LeetCode Java Solutions

     

Hi 👋, I'm Chee Hwa and welcome to LeetCode Solutions in Java.

As I'm new to LeetCode and programming in general, I'm practicing everyday and hope to share my solutions and learnings with you.

• Exclusively on Java
• Beginner problems
• Daily Challenge if within my ability

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Latest Solutions

Date	Problem	Difficulty	Solution	Topics
January 9th	130. Surrounded Regions	$\text{\color{Dandelion}Medium}$	Breadth-First Search	Array, Breadth-First Search, Matrix,
January 8th	130. Surrounded Regions	$\text{\color{Dandelion}Medium}$	Depth-First Search	Array, Depth-First Search, Matrix,
January 7th	922. Sort Array By Parity II	$\text{\color{TealBlue}Easy}$	Sorting & Two Pointers	Array, Sorting, Two Pointers
January 6th	905. Sort Array By Parity	$\text{\color{TealBlue}Easy}$	Sorting & Two Pointers	Array, Sorting, Two Pointers
January 5th	200. Number of Islands	$\text{\color{Dandelion}Medium}$	Union Find	Array, Matrix, Union Find

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Topics

The problem sets and solutions are grouped into Topics as stated in LeetCode.

• Array
• Backtracking
• Binary Search
• Bit Manipulation
• Brainteaser
• Breadth-First Search
• Counting
• Data Stream
• Depth-First Search
• Design
• Dynamic Programming
• Enumeration
• Geometry
• Graph
• Greedy
• Hash Table
• Heap (Priority Queue)
• Interactive
• Linked List
• Math
• Matrix
• Memoization
• Monotonic Stack
• Number Theory
• Prefix Sum
• Recursion
• Simulation
• Sorting
• Stack
• String
• Two Pointers
• Union Find

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Array

#	Problem	Solution	Difficulty	Topics	Note
1	Two Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table
26	Remove Duplicates from Sorted Array	Java	$\text{\color{TealBlue}Easy}$	Array, Two Pointers
27	Remove Element	Java	$\text{\color{TealBlue}Easy}$	Array, Two Pointers
36	Valid Sudoku	Java without or with Hash Table	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Matrix
37	Sudoku Solver	Java	$\text{\color{Red}Hard}$	Array, Backtracking, Matrix
49	Group Anagrams	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting, String
73	Set Matrix Zeroes	Java with Array, Hash Table or In-Place	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Matrix
79	Word Search	Java	$\text{\color{Dandelion}Medium}$	Array, Backtracking, Matrix
80	Remove Duplicates from Sorted Array II	Java	$\text{\color{Dandelion}Medium}$	Array, Two Pointers
88	Merge Sorted Array	Java with Sorting or Two Pointers	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers
130	Surrounded Regions	Java with Breadth-First Search or Depth-First Search	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix,
200	Number of Islands	Java with Breadth-First Search, Depth-First Search or Union Find	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix, Union Find
238	Product of Array Except Self	Java	$\text{\color{Dandelion}Medium}$	Array, Prefix Sum
283	Move Zeroes	Java	$\text{\color{TealBlue}Easy}$	Array, Two Pointers
334	Increasing Triplet Subsequence	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy
368	Largest Divisible Subset	Java	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Math, Sorting
393	UTF-8 Validation	Java without or with Bit Manipulation	$\text{\color{Dandelion}Medium}$	Array, Bit Manipulation
485	Maximum Consecutive Ones	Java	$\text{\color{TealBlue}Easy}$	Array
496	Next Greater Element I	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Monotonic Stack, Stack
503	Next Greater Element II	Java	$\text{\color{Dandelion}Medium}$	Array, Monotonic Stack, Stack
523	Continuous Subarray Sum	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Math, Prefix Sum
560	Subarray Sum Equals K	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Prefix Sum
609	Find Duplicate File in System	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, String
645	Set Mismatch	Java with Bit Manipulation, Hash Table or Sorting	$\text{\color{TealBlue}Easy}$	Array, Bit Manipulation, Hash Table, Sorting
746	Min Cost Climbing Stairs	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Array, Dynamic Programming
766	Toeplitz Matrix	Java	$\text{\color{TealBlue}Easy}$	Array, Matrix	Follow up: One Row at a Time and One Column at a Time
812	Largest Triangle Area	Java	$\text{\color{TealBlue}Easy}$	Array, Geometry, Math
835	Image Overlap	Java with Bit Manipulation or Hash Table	$\text{\color{Dandelion}Medium}$	Array, Matrix
905	Sort Array By Parity	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers
922	Sort Array By Parity II	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers
929	Unique Email Addresses	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, String
941	Valid Mountain Array	Java	$\text{\color{TealBlue}Easy}$	Array
948	Bag of Tokens	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy, Sorting, Two Pointers
974	Subarray Sums Divisible by K	Java with Counting or Hash Table	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Prefix Sum
976	Largest Perimeter Triangle	Java	$\text{\color{TealBlue}Easy}$	Array, Greedy, Math, Sorting
977	Squares of a Sorted Array	Java with Sorting or Two Pointers	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers
985	Sum of Even Numbers After Queries	Java	$\text{\color{Dandelion}Medium}$	Array, Simulation
1037	Valid Boomerang	Java	$\text{\color{TealBlue}Easy}$	Array, Geometry, Math
1089	Duplicate Zeros	Java without or with Two Pointers	$\text{\color{TealBlue}Easy}$	Array, Two Pointers
1239	Maximum Length of a Concatenated String with Unique Characters	Java with Backtracking or Bit Manipulation	$\text{\color{Dandelion}Medium}$	Array, Backtracking, Bit Manipulation, String
1295	Find Numbers with Even Number of Digits	Java	$\text{\color{TealBlue}Easy}$	Array
1299	Replace Elements with Greatest Element on Right Side	Java	$\text{\color{TealBlue}Easy}$	Array
1337	The K Weakest Rows in a Matrix	Java with Heap (Priority Queue) or Sorting	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Heap (Priority Queue), Matrix, Sorting
1480	Running Sum of 1D Array	Java	$\text{\color{TealBlue}Easy}$	Array, Prefix Sum
1497	Check If Array Pairs Are Divisible by k	Java with Counting or Hash Table	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table
1578	Minimum Time to Make Rope Colorful	Java	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Greedy, String
1672	Richest Customer Wealth	Java	$\text{\color{TealBlue}Easy}$	Array, Matrix
1679	Max Number of K-Sum Pairs	Java with Hash Table or Sorting & Two Pointers	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting, Two Pointers
1711	Count Good Meals	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table
1834	Single-Threaded CPU	Java	$\text{\color{Dandelion}Medium}$	Array, Heap (Priority Queue), Sorting
1865	Finding Pairs With a Certain Sum	Java	$\text{\color{Dandelion}Medium}$	Array, Design, Hash Table
1979	Find Greatest Common Divisor of Array	Java	$\text{\color{TealBlue}Easy}$	Array, Math, Number Theory
1995	Count Special Quadruplets	Java	$\text{\color{TealBlue}Easy}$	Array, Enumeration	Solution Using Hash Table
2007	Find Original Array From Doubled Array	Java with Hash Table, Queue or Counting Array	$\text{\color{Dandelion}Medium}$	Array, Greedy, Hash Table, Sorting
2023	Number of Pairs of Strings With Concatenation Equal to Target	Java	$\text{\color{Dandelion}Medium}$	Array, String
2131	Longest Palindrome by Concatenating Two Letter Words	Java with Counting or Hash Table	$\text{\color{Dandelion}Medium}$	Array, Counting, Greedy, Hash Table, String
2133	Check if Every Row and Column Contains All Numbers	Java without or with Hash Table	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Matrix
2136	Earliest Possible Day of Full Bloom	Java	$\text{\color{Red}Hard}$	Array, Greedy, Sorting
2176	Count Equal and Divisible Pairs in an Array	Java	$\text{\color{TealBlue}Easy}$	Array
2389	Longest Subsequence With Limited Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Greedy, Prefix Sum, Sorting
2395	Find Subarrays With Equal Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table
2399	Check Distances Between Same Letters	Java without or with Hash Table	$\text{\color{TealBlue}Easy}$	Array, Hash Table, String
2438	Range Product Queries of Powers	Java	$\text{\color{Dandelion}Medium}$	Array, Bit Manipulation, Prefix Sum
2452	Words Within Two Edits of Dictionary	Java	$\text{\color{Dandelion}Medium}$	Array, String
2453	Destroy Sequential Targets	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table
2454	Next Greater Element IV	Java	$\text{\color{Red}Hard}$	Array, Monotonic Stack, Stack

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Backtracking

#	Problem	Solution	Difficulty	Topics	Note
37	Sudoku Solver	Java	$\text{\color{Red}Hard}$	Array, Backtracking, Matrix
79	Word Search	Java	$\text{\color{Dandelion}Medium}$	Array, Backtracking, Matrix
797	All Paths From Source to Target	Java	$\text{\color{Dandelion}Medium}$	Backtracking, Breadth-First Search, Depth-First Search, Graph	Solution Using Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)
967	Numbers With Same Consecutive Differences	Java	$\text{\color{Dandelion}Medium}$	Backtracking, Breadth-First Search	Solution Using Breadth-First Search
1239	Maximum Length of a Concatenated String with Unique Characters	Java	$\text{\color{Dandelion}Medium}$	Array, Backtracking, Bit Manipulation, String	Solution Using Bit Manipulation

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Binary Search

#	Problem	Solution	Difficulty	Topics	Note
374	Guess Number Higher or Lower	Java	$\text{\color{TealBlue}Easy}$	Binary Search, Interactive
1337	The K Weakest Rows in a Matrix	Java with Heap (Priority Queue) or Sorting	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Heap (Priority Queue), Matrix, Sorting	Solution Using Linear Search
2389	Longest Subsequence With Limited Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Greedy, Prefix Sum, Sorting

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Bit Manipulation

#	Problem	Solution	Difficulty	Topics	Note
29	Divide Two Integers	Java	$\text{\color{Dandelion}Medium}$	Bit Manipulation, Math
371	Sum of Two Integers	Java	$\text{\color{Dandelion}Medium}$	Bit Manipulation, Math
393	UTF-8 Validation	Java	$\text{\color{Dandelion}Medium}$	Array, Bit Manipulation
645	Set Mismatch	Java	$\text{\color{TealBlue}Easy}$	Array, Bit Manipulation, Hash Table, Sorting	Solution Using Hash Table or Sorting
1239	Maximum Length of a Concatenated String with Unique Characters	Java	$\text{\color{Dandelion}Medium}$	Array, Backtracking, Bit Manipulation, String	Solution Using Backtracking
1342	Number of Steps to Reduce a Number to Zero	Java	$\text{\color{TealBlue}Easy}$	Bit Manipulation, Math	Solution Using Math
1486	XOR Operation in an Array	Java	$\text{\color{TealBlue}Easy}$	Bit Manipulation, Math
1680	Concatenation of Consecutive Binary Numbers	Java	$\text{\color{Dandelion}Medium}$	Bit Manipulation, Math, Simulation	Solution Using Math
2438	Range Product Queries of Powers	Java	$\text{\color{Dandelion}Medium}$	Array, Bit Manipulation, Prefix Sum

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Brainteaser

#	Problem	Solution	Difficulty	Topics	Note
1033	Moving Stones Until Consecutive	Java	$\text{\color{Dandelion}Medium}$	Brainteaser, Math

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Breadth-First Search

#	Problem	Solution	Difficulty	Topics	Note
130	Surrounded Regions	Java	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix,	Solution Using Depth-First Search
200	Number of Islands	Java	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix, Union Find	Solution Using Depth-First Search or Union Find
797	All Paths From Source to Target	Java	$\text{\color{Dandelion}Medium}$	Backtracking, Breadth-First Search, Depth-First Search, Graph	Solution Using Depth-First Search (Backtracking), Depth-First Search (Iterative) or Depth-First Search (Recursive)
967	Numbers With Same Consecutive Differences	Java	$\text{\color{Dandelion}Medium}$	Backtracking, Breadth-First Search	Solution Using Backtracking
1971	Find if Path Exists in Graph	Java	$\text{\color{TealBlue}Easy}$	Breadth-First Search, Depth-First Search, Graph, Union Find	Solution Using Depth-First Search (Iterative), Depth-First Search (Recursive) or Union Find

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Counting

#	Problem	Solution	Difficulty	Topics	Note
383	Ransom Note	Java	$\text{\color{TealBlue}Easy}$	Counting, Hash Table, String	Solution Using Hash Table
1497	Check If Array Pairs Are Divisible by k	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table	Solution Using Hash Table
2131	Longest Palindrome by Concatenating Two Letter Words	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Greedy, Hash Table, String	Solution Using Hash Table
2351	First Letter to Appear Twice	Java	$\text{\color{TealBlue}Easy}$	Counting, Hash Table, String	Solution Using Hash Table
2453	Destroy Sequential Targets	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table

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Data Stream

#	Problem	Solution	Difficulty	Topics	Note
295	Find Median from Data Stream	Java	$\text{\color{Red}Hard}$	Data Stream, Design, Heap (Priority Queue), Sorting, Two Pointers

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Depth-First Search

or Union Find

#	Problem	Solution	Difficulty	Topics	Note
130	Surrounded Regions	Java	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix,	Solution Using Breadth-First Search
200	Number of Islands	Java	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix, Union Find	Solution Using Breadth-First Search or Union Find
797	All Paths From Source to Target	Java with Depth-First Search (Backtracking), Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Backtracking, Breadth-First Search, Depth-First Search, Graph	Solution Using Breadth-First Search
1971	Find if Path Exists in Graph	Java with Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{TealBlue}Easy}$	Breadth-First Search, Depth-First Search, Graph, Union Find	Solution Using Breadth-First Search

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Design

#	Problem	Solution	Difficulty	Topics	Note
295	Find Median from Data Stream	Java	$\text{\color{Red}Hard}$	Data Stream, Design, Heap (Priority Queue), Sorting, Two Pointers
707	Design Linked List	Java with Singly Linked List or Doubly Linked List	$\text{\color{Dandelion}Medium}$	Design, Linked List
1865	Finding Pairs With a Certain Sum	Java	$\text{\color{Dandelion}Medium}$	Array, Design, Hash Table

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Dynamic Programming

#	Problem	Solution	Difficulty	Topics	Note
70	Climbing Stairs	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Dynamic Programming, Math, Memoization
264	Ugly Number II	Java	$\text{\color{Dandelion}Medium}$	Dynamic Programming, Hash Table, Heap (Priority Queue), Math	Solution Using Heap (Priority Queue)
368	Largest Divisible Subset	Java	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Math, Sorting
509	Fibonacci Number	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Dynamic Programming, Math, Memoization, Recursion
746	Min Cost Climbing Stairs	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Array, Dynamic Programming
1578	Minimum Time to Make Rope Colorful	Java	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Greedy, String

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Enumeration

#	Problem	Solution	Difficulty	Topics	Note
1995	Count Special Quadruplets	Java	$\text{\color{TealBlue}Easy}$	Array, Enumeration	Solution Using Hash Table
2240	Number of Ways to Buy Pens and Pencils	Java	$\text{\color{Dandelion}Medium}$	Enumeration, Math
2437	Number of Valid Clock Times	Java	$\text{\color{TealBlue}Easy}$	Enumeration, String

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Geometry

#	Problem	Solution	Difficulty	Topics	Note
223	Rectangle Area	Java	$\text{\color{Dandelion}Medium}$	Geometry, Math
593	Valid Square	Java	$\text{\color{Dandelion}Medium}$	Geometry, Math
812	Largest Triangle Area	Java	$\text{\color{TealBlue}Easy}$	Array, Geometry, Math
836	Rectangle Overlap	Java	$\text{\color{TealBlue}Easy}$	Geometry, Math
1037	Valid Boomerang	Java	$\text{\color{TealBlue}Easy}$	Array, Geometry, Math

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Graph

#	Problem	Solution	Difficulty	Topics	Note
797	All Paths From Source to Target	Java with Breadth-First Search, Depth-First Search (Backtracking), Depth-First Search (Iterative) or Depth-First Search (Recursive)	$\text{\color{Dandelion}Medium}$	Backtracking, Breadth-First Search, Depth-First Search, Graph
1971	Find if Path Exists in Graph	Java with Breadth-First Search, Depth-First Search (Iterative), Depth-First Search (Recursive) or Union Find	$\text{\color{TealBlue}Easy}$	Breadth-First Search, Depth-First Search, Graph, Union Find

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Greedy

#	Problem	Solution	Difficulty	Topics	Note
334	Increasing Triplet Subsequence	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy
948	Bag of Tokens	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy, Sorting, Two Pointers
976	Largest Perimeter Triangle	Java	$\text{\color{TealBlue}Easy}$	Array, Greedy, Math, Sorting
1323	Maximum 69 Number	Java with Math or String	$\text{\color{TealBlue}Easy}$	Greedy, Math
1328	Break a Palindrome	Java	$\text{\color{Dandelion}Medium}$	Greedy, String
1578	Minimum Time to Make Rope Colorful	Java	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Greedy, String
2007	Find Original Array From Doubled Array	Java with Hash Table, Queue or Counting Array	$\text{\color{Dandelion}Medium}$	Array, Greedy, Hash Table, Sorting
2131	Longest Palindrome by Concatenating Two Letter Words	Java with Counting or Hash Table	$\text{\color{Dandelion}Medium}$	Array, Counting, Greedy, Hash Table, String
2136	Earliest Possible Day of Full Bloom	Java	$\text{\color{Red}Hard}$	Array, Greedy, Sorting
2389	Longest Subsequence With Limited Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Greedy, Prefix Sum, Sorting

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Hash Table

#	Problem	Solution	Difficulty	Topics	Note
1	Two Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table
12	Integer to Roman	Java	$\text{\color{Dandelion}Medium}$	Hash Table, Math, String
13	Roman to Integer	Java	$\text{\color{TealBlue}Easy}$	Hash Table, Math, String	Solution Without Hash Table
36	Valid Sudoku	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Matrix	Solution without Hash Table
49	Group Anagrams	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting, String
73	Set Matrix Zeroes	Java with Array, Hash Table or In-Place	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Matrix
264	Ugly Number II	Java with Dynamic Programming or Heap (Priority Queue)	$\text{\color{Dandelion}Medium}$	Dynamic Programming, Hash Table, Heap (Priority Queue), Math
290	Word Pattern	Java	$\text{\color{TealBlue}Easy}$	Hash Table, String
383	Ransom Note	Java	$\text{\color{TealBlue}Easy}$	Counting, Hash Table, String	Solution Using Counting Array
496	Next Greater Element I	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Monotonic Stack, Stack
523	Continuous Subarray Sum	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Math, Prefix Sum
560	Subarray Sum Equals K	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Prefix Sum
609	Find Duplicate File in System	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, String
645	Set Mismatch	Java	$\text{\color{TealBlue}Easy}$	Array, Bit Manipulation, Hash Table, Sorting	Solution Using Bit Manipulation or Sorting
929	Unique Email Addresses	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, String
974	Subarray Sums Divisible by K	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Prefix Sum	Solution Using Counting
1497	Check If Array Pairs Are Divisible by k	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table	Solution Using Counting
1679	Max Number of K-Sum Pairs	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting, Two Pointers	Solution Using Sorting & Two Pointers
1711	Count Good Meals	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table
1832	Check if the Sentence is Pangram	Java	$\text{\color{TealBlue}Easy}$	Hash Table, String	Solution Without Hash Table
1865	Finding Pairs With a Certain Sum	Java	$\text{\color{Dandelion}Medium}$	Array, Design, Hash Table
1935	Maximum Number of Words You Can Type	Java	$\text{\color{TealBlue}Easy}$	Hash Table, String
2007	Find Original Array From Doubled Array	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy, Hash Table, Sorting	Solution Using Queue or Counting Array
2131	Longest Palindrome by Concatenating Two Letter Words	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Greedy, Hash Table, String	Solution Using Counting
2133	Check if Every Row and Column Contains All Numbers	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Matrix	Solution Without Hash Table
2351	First Letter to Appear Twice	Java	$\text{\color{TealBlue}Easy}$	Counting, Hash Table, String	Solution Using Counting
2395	Find Subarrays With Equal Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table
2399	Check Distances Between Same Letters	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, String	Solution Without Hash Table
2451	Odd String Difference	Java	$\text{\color{TealBlue}Easy}$	Hash Table, Math, String	Solution Without Hash Table
2453	Destroy Sequential Targets	Java	$\text{\color{Dandelion}Medium}$	Array, Counting, Hash Table

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Heap (Priority Queue)

#	Problem	Solution	Difficulty	Topics	Note
264	Ugly Number II	Java	$\text{\color{Dandelion}Medium}$	Dynamic Programming, Hash Table, Heap (Priority Queue), Math	Solution Using Dynamic Programming
295	Find Median from Data Stream	Java	$\text{\color{Red}Hard}$	Data Stream, Design, Heap (Priority Queue), Sorting, Two Pointers
1337	The K Weakest Rows in a Matrix	Java	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Heap (Priority Queue), Matrix, Sorting	Solution Using Sorting
1834	Single-Threaded CPU	Java	$\text{\color{Dandelion}Medium}$	Array, Heap (Priority Queue), Sorting

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Interactive

#	Problem	Solution	Difficulty	Topics	Note
374	Guess Number Higher or Lower	Java	$\text{\color{TealBlue}Easy}$	Binary Search, Interactive

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Linked List

#	Problem	Solution	Difficulty	Topics	Note
19	Remove Nth Node From End of List	Java with One Pass or Two Passes	$\text{\color{Dandelion}Medium}$	Linked List, Two Pointers
707	Design Linked List	Java with Singly Linked List or Doubly Linked List	$\text{\color{Dandelion}Medium}$	Design, Linked List
876	Middle of the Linked List	Java	$\text{\color{TealBlue}Easy}$	Linked List, Two Pointers
2095	Delete the Middle Node of a Linked List	Java	$\text{\color{Dandelion}Medium}$	Linked List, Two Pointers

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Math

#	Problem	Solution	Difficulty	Topics	Note
12	Integer to Roman	Java	$\text{\color{Dandelion}Medium}$	Hash Table, Math, String
13	Roman to Integer	Java	$\text{\color{TealBlue}Easy}$	Hash Table, Math, String	Solution Using Hash Table
29	Divide Two Integers	Java	$\text{\color{Dandelion}Medium}$	Bit Manipulation, Math
70	Climbing Stairs	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Dynamic Programming, Math, Memoization
223	Rectangle Area	Java	$\text{\color{Dandelion}Medium}$	Geometry, Math
263	Ugly Number	Java	$\text{\color{TealBlue}Easy}$	Math
264	Ugly Number II	Java with Dynamic Programming or Heap (Priority Queue)	$\text{\color{Dandelion}Medium}$	Dynamic Programming, Hash Table, Heap (Priority Queue), Math
368	Largest Divisible Subset	Java	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Math, Sorting
371	Sum of Two Integers	Java	$\text{\color{Dandelion}Medium}$	Bit Manipulation, Math
412	Fizz Buzz	Java	$\text{\color{TealBlue}Easy}$	Math, Simulation, String
492	Construct the Rectangle	Java	$\text{\color{TealBlue}Easy}$	Math
509	Fibonacci Number	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Dynamic Programming, Math, Memoization, Recursion
523	Continuous Subarray Sum	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Math, Prefix Sum
556	Next Greater Element III	Java	$\text{\color{Dandelion}Medium}$	Math, String, Two Pointers
593	Valid Square	Java	$\text{\color{Dandelion}Medium}$	Geometry, Math
812	Largest Triangle Area	Java	$\text{\color{TealBlue}Easy}$	Array, Geometry, Math
836	Rectangle Overlap	Java	$\text{\color{TealBlue}Easy}$	Geometry, Math
976	Largest Perimeter Triangle	Java	$\text{\color{TealBlue}Easy}$	Array, Greedy, Math, Sorting
1033	Moving Stones Until Consecutive	Java	$\text{\color{Dandelion}Medium}$	Brainteaser, Math
1037	Valid Boomerang	Java	$\text{\color{TealBlue}Easy}$	Array, Geometry, Math
1071	Greatest Common Divisor of Strings	Java	$\text{\color{TealBlue}Easy}$	Math, String
1103	Distribute Candies to People	Java	$\text{\color{TealBlue}Easy}$	Math, Simulation
1154	Day of the Year	Java	$\text{\color{TealBlue}Easy}$	Math, String
1323	Maximum 69 Number	Java	$\text{\color{TealBlue}Easy}$	Greedy, Math	Solution Using String
1342	Number of Steps to Reduce a Number to Zero	Java	$\text{\color{TealBlue}Easy}$	Bit Manipulation, Math	Solution Using Bit Manipulation
1486	XOR Operation in an Array	Java	$\text{\color{TealBlue}Easy}$	Bit Manipulation, Math
1680	Concatenation of Consecutive Binary Numbers	Java	$\text{\color{Dandelion}Medium}$	Bit Manipulation, Math, Simulation	Solution Using Bit Manipulation
1979	Find Greatest Common Divisor of Array	Java	$\text{\color{TealBlue}Easy}$	Array, Math, Number Theory
2169	Count Operations to Obtain Zero	Java	$\text{\color{TealBlue}Easy}$	Math, Simulation
2240	Number of Ways to Buy Pens and Pencils	Java	$\text{\color{Dandelion}Medium}$	Enumeration, Math
2413	Smallest Even Multiple	Java	$\text{\color{TealBlue}Easy}$	Math, Number Theory
2451	Odd String Difference	Java without or with Hash Table	$\text{\color{TealBlue}Easy}$	Hash Table, Math, String

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Matrix

#	Problem	Solution	Difficulty	Topics	Note
36	Valid Sudoku	Java without or with Hash Table	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Matrix
37	Sudoku Solver	Java	$\text{\color{Red}Hard}$	Array, Backtracking, Matrix
73	Set Matrix Zeroes	Java with Array, Hash Table or In-Place	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Matrix
79	Word Search	Java	$\text{\color{Dandelion}Medium}$	Array, Backtracking, Matrix
130	Surrounded Regions	Java with Breadth-First Search or Depth-First Search	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix,
200	Number of Islands	Java with Breadth-First Search, Depth-First Search or Union Find	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix, Union Find
766	Toeplitz Matrix	Java	$\text{\color{TealBlue}Easy}$	Array, Matrix	Follow up: One Row at a Time and One Column at a Time
835	Image Overlap	Java with Bit Manipulation or Hash Table	$\text{\color{Dandelion}Medium}$	Array, Matrix
1337	The K Weakest Rows in a Matrix	Java with Heap (Priority Queue) or Sorting	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Heap (Priority Queue), Matrix, Sorting
1672	Richest Customer Wealth	Java	$\text{\color{TealBlue}Easy}$	Array, Matrix
2133	Check if Every Row and Column Contains All Numbers	Java without or with Hash Table	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Matrix

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Memoization

#	Problem	Solution	Difficulty	Topics	Note
70	Climbing Stairs	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Dynamic Programming, Math, Memoization
509	Fibonacci Number	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Dynamic Programming, Math, Memoization, Recursion

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Monotonic Stack

#	Problem	Solution	Difficulty	Topics	Note
496	Next Greater Element I	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Monotonic Stack, Stack
503	Next Greater Element II	Java	$\text{\color{Dandelion}Medium}$	Array, Monotonic Stack, Stack
2454	Next Greater Element IV	Java	$\text{\color{Red}Hard}$	Array, Monotonic Stack, Stack

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Number Theory

#	Problem	Solution	Difficulty	Topics	Note
1979	Find Greatest Common Divisor of Array	Java	$\text{\color{TealBlue}Easy}$	Array, Math, Number Theory
2413	Smallest Even Multiple	Java	$\text{\color{TealBlue}Easy}$	Math, Number Theory

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Prefix Sum

#	Problem	Solution	Difficulty	Topics	Note
238	Product of Array Except Self	Java	$\text{\color{Dandelion}Medium}$	Array, Prefix Sum
523	Continuous Subarray Sum	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Math, Prefix Sum
560	Subarray Sum Equals K	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Prefix Sum
974	Subarray Sums Divisible by K	Java with Counting or Hash Table	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Prefix Sum
1480	Running Sum of 1D Array	Java	$\text{\color{TealBlue}Easy}$	Array, Prefix Sum
2389	Longest Subsequence With Limited Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Greedy, Prefix Sum, Sorting
2438	Range Product Queries of Powers	Java	$\text{\color{Dandelion}Medium}$	Array, Bit Manipulation, Prefix Sum

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Recursion

#	Problem	Solution	Difficulty	Topics	Note
509	Fibonacci Number	Java with Dynamic Programming - Tabulation or Memoization	$\text{\color{TealBlue}Easy}$	Dynamic Programming, Math, Memoization, Recursion

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Simulation

#	Problem	Solution	Difficulty	Topics	Note
412	Fizz Buzz	Java	$\text{\color{TealBlue}Easy}$	Math, Simulation, String
985	Sum of Even Numbers After Queries	Java	$\text{\color{Dandelion}Medium}$	Array, Simulation
1103	Distribute Candies to People	Java	$\text{\color{TealBlue}Easy}$	Math, Simulation
1680	Concatenation of Consecutive Binary Numbers	Java with Bit Manipulation or Math	$\text{\color{Dandelion}Medium}$	Bit Manipulation, Math, Simulation
2169	Count Operations to Obtain Zero	Java	$\text{\color{TealBlue}Easy}$	Math, Simulation

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Sorting

#	Problem	Solution	Difficulty	Topics	Note
49	Group Anagrams	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting, String
88	Merge Sorted Array	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers	Solution Using Two Pointers
295	Find Median from Data Stream	Java with Heap (Priority Queue)	$\text{\color{Red}Hard}$	Data Stream, Design, Heap (Priority Queue), Sorting, Two Pointers	Use of Sorting & Two Pointers resulted in TLE (Time Limit Exceed) for large datasets.
645	Set Mismatch	Java	$\text{\color{TealBlue}Easy}$	Array, Bit Manipulation, Hash Table, Sorting	Solution Using Bit Manipulation or Hash Table
905	Sort Array By Parity	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers
922	Sort Array By Parity II	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers
948	Bag of Tokens	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy, Sorting, Two Pointers
976	Largest Perimeter Triangle	Java	$\text{\color{TealBlue}Easy}$	Array, Greedy, Math, Sorting
977	Squares of a Sorted Array	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers	Solution Using Two Pointers
1337	The K Weakest Rows in a Matrix	Java	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Heap (Priority Queue), Matrix, Sorting	Solution Using Heap (Priority Queue)
1679	Max Number of K-Sum Pairs	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting, Two Pointers	Solution Using Hash Table
1834	Single-Threaded CPU	Java	$\text{\color{Dandelion}Medium}$	Array, Heap (Priority Queue), Sorting
2007	Find Original Array From Doubled Array	Java with Hash Table or Queue	$\text{\color{Dandelion}Medium}$	Array, Greedy, Hash Table, Sorting	Solution Using Counting Array
2136	Earliest Possible Day of Full Bloom	Java	$\text{\color{Red}Hard}$	Array, Greedy, Sorting
2389	Longest Subsequence With Limited Sum	Java	$\text{\color{TealBlue}Easy}$	Array, Binary Search, Greedy, Prefix Sum, Sorting

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Stack

#	Problem	Solution	Difficulty	Topics	Note
496	Next Greater Element I	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, Monotonic Stack, Stack
503	Next Greater Element II	Java	$\text{\color{Dandelion}Medium}$	Array, Monotonic Stack, Stack
1047	Remove All Adjacent Duplicates In String	Java	$\text{\color{TealBlue}Easy}$	Stack, String
1544	Make The String Great	Java	$\text{\color{TealBlue}Easy}$	Stack, String
2454	Next Greater Element IV	Java	$\text{\color{Red}Hard}$	Array, Monotonic Stack, Stack

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String

#	Problem	Solution	Difficulty	Topics	Note
12	Integer to Roman	Java	$\text{\color{Dandelion}Medium}$	Hash Table, Math, String
13	Roman to Integer	Java	$\text{\color{TealBlue}Easy}$	Hash Table, Math, String	Solution Using Hash Table
38	Count and Say	Java with Iterative or Recursive Algorithms	$\text{\color{Dandelion}Medium}$	String
49	Group Anagrams	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting, String
151	Reverse Words in a String	Java with String or Collections Implementations	$\text{\color{Dandelion}Medium}$	String, Two Pointers	Solution Using Two Pointers
290	Word Pattern	Java	$\text{\color{TealBlue}Easy}$	Hash Table, String
344	Reverse String	Java	$\text{\color{TealBlue}Easy}$	String, Two Pointers
345	Reverse Vowels of a String	Java	$\text{\color{TealBlue}Easy}$	String, Two Pointers
368	Largest Divisible Subset	Java	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Math, Sorting
383	Ransom Note	Java with Counting Array or Hash Table	$\text{\color{TealBlue}Easy}$	Counting, Hash Table, String
412	Fizz Buzz	Java	$\text{\color{TealBlue}Easy}$	Math, Simulation, String
556	Next Greater Element III	Java	$\text{\color{Dandelion}Medium}$	Math, String, Two Pointers
609	Find Duplicate File in System	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, String
929	Unique Email Addresses	Java	$\text{\color{TealBlue}Easy}$	Array, Hash Table, String
1047	Remove All Adjacent Duplicates In String	Java	$\text{\color{TealBlue}Easy}$	Stack, String
1071	Greatest Common Divisor of Strings	Java	$\text{\color{TealBlue}Easy}$	Math, String
1154	Day of the Year	Java	$\text{\color{TealBlue}Easy}$	Math, String
1239	Maximum Length of a Concatenated String with Unique Characters	Java with Backtracking or Bit Manipulation	$\text{\color{Dandelion}Medium}$	Array, Backtracking, Bit Manipulation, String
1328	Break a Palindrome	Java	$\text{\color{Dandelion}Medium}$	Greedy, String
1544	Make The String Great	Java	$\text{\color{TealBlue}Easy}$	Stack, String
1578	Minimum Time to Make Rope Colorful	Java	$\text{\color{Dandelion}Medium}$	Array, Dynamic Programming, Greedy, String
1592	Rearrange Spaces Between Words	Java	$\text{\color{TealBlue}Easy}$	String
1832	Check if the Sentence is Pangram	Java	$\text{\color{TealBlue}Easy}$	Hash Table, String	Solution Using Hash Table
1935	Maximum Number of Words You Can Type	Java	$\text{\color{TealBlue}Easy}$	Hash Table, String
2023	Number of Pairs of Strings With Concatenation Equal to Target	Java	$\text{\color{Dandelion}Medium}$	Array, String
2131	Longest Palindrome by Concatenating Two Letter Words	Java with Counting or Hash Table	$\text{\color{Dandelion}Medium}$	Array, Counting, Greedy, Hash Table, String
2315	Count Asterisks	Java	$\text{\color{TealBlue}Easy}$	String
2351	First Letter to Appear Twice	Java with Counting or Hash Table	$\text{\color{TealBlue}Easy}$	Counting, Hash Table, String
2399	Check Distances Between Same Letters	Java without or with Hash Table	$\text{\color{TealBlue}Easy}$	Array, Hash Table, String
2437	Number of Valid Clock Times	Java	$\text{\color{TealBlue}Easy}$	Enumeration, String
2451	Odd String Difference	Java without or with Hash Table	$\text{\color{TealBlue}Easy}$	Hash Table, Math, String
2452	Words Within Two Edits of Dictionary	Java	$\text{\color{Dandelion}Medium}$	Array, String

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Two Pointers

#	Problem	Solution	Difficulty	Topics	Note
19	Remove Nth Node From End of List	Java with One Pass or Two Passes	$\text{\color{Dandelion}Medium}$	Linked List, Two Pointers
26	Remove Duplicates from Sorted Array	Java	$\text{\color{TealBlue}Easy}$	Array, Two Pointers
27	Remove Element	Java	$\text{\color{TealBlue}Easy}$	Array, Two Pointers
80	Remove Duplicates from Sorted Array II	Java	$\text{\color{Dandelion}Medium}$	Array, Two Pointers
88	Merge Sorted Array	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers	Solution Using Sorting
151	Reverse Words in a String	Java	$\text{\color{Dandelion}Medium}$	String, Two Pointers	Solution Using Collections
283	Move Zeroes	Java	$\text{\color{TealBlue}Easy}$	Array, Two Pointers
295	Find Median from Data Stream	Java with Heap (Priority Queue)	$\text{\color{Red}Hard}$	Data Stream, Design, Heap (Priority Queue), Sorting, Two Pointers	Use of Sorting & Two Pointers resulted in TLE (Time Limit Exceed) for large datasets.
344	Reverse String	Java	$\text{\color{TealBlue}Easy}$	String, Two Pointers
345	Reverse Vowels of a String	Java	$\text{\color{TealBlue}Easy}$	String, Two Pointers
556	Next Greater Element III	Java	$\text{\color{Dandelion}Medium}$	Math, String, Two Pointers
876	Middle of the Linked List	Java	$\text{\color{TealBlue}Easy}$	Linked List, Two Pointers
905	Sort Array By Parity	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers
922	Sort Array By Parity II	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers
948	Bag of Tokens	Java	$\text{\color{Dandelion}Medium}$	Array, Greedy, Sorting, Two Pointers
977	Squares of a Sorted Array	Java	$\text{\color{TealBlue}Easy}$	Array, Sorting, Two Pointers	Solution Using Sorting
1089	Duplicate Zeros	Java	$\text{\color{TealBlue}Easy}$	Array, Two Pointers	Solution Without Two Pointers
1679	Max Number of K-Sum Pairs	Java	$\text{\color{Dandelion}Medium}$	Array, Hash Table, Sorting, Two Pointers	Solution Using Hash Table
2095	Delete the Middle Node of a Linked List	Java	$\text{\color{Dandelion}Medium}$	Linked List, Two Pointers

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Union Find

#	Problem	Solution	Difficulty	Topics	Note
200	Number of Islands	Java	$\text{\color{Dandelion}Medium}$	Array, Breadth-First Search, Depth-First Search, Matrix, Union Find	Solution Using Breadth-First Search or Depth-First Search
1971	Find if Path Exists in Graph	Java	$\text{\color{TealBlue}Easy}$	Breadth-First Search, Depth-First Search, Graph, Union Find	Solution Using Breadth-First Search, Depth-First Search (Iterative) or Depth-First Search (Recursive)

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