src/main/java/com/fishercoder/solutions/_438.java

package com.fishercoder.solutions;

import java.util.ArrayList;
import java.util.List;

/**
 * 438. Find All Anagrams in a String
 *
 * Given a string s and a non-empty string p, find all the start indices of p's anagrams in s.
 Strings consists of lowercase English letters only and the length of both strings s and p will not be larger than 20,100.

 The order of output does not matter.

 Example 1:
 Input:
 s: "cbaebabacd" p: "abc"
 Output:
 [0, 6]

 Explanation:
 The substring with start index = 0 is "cba", which is an anagram of "abc".
 The substring with start index = 6 is "bac", which is an anagram of "abc".

 Example 2:
 Input:
 s: "abab" p: "ab"
 Output:
 [0, 1, 2]

 Explanation:
 The substring with start index = 0 is "ab", which is an anagram of "ab".
 The substring with start index = 1 is "ba", which is an anagram of "ab".
 The substring with start index = 2 is "ab", which is an anagram of "ab".*/

public class _438 {
    public static class Solution1 {
        /**
         * O(m*n) solution, my original and most intuitive one, but sort of brute force, when m is close to n, it becomes O(n^2) runtime complexity.
         */
        public List<Integer> findAnagrams(String s, String p) {
            List<Integer> result = new ArrayList();
            for (int i = 0; i <= s.length() - p.length(); i++) {
                if (isAnagram(s.substring(i, i + p.length()), p)) {
                    result.add(i);
                }
            }
            return result;
        }

        private boolean isAnagram(String s, String p) {
            int[] c = new int[26];
            for (int i = 0; i < s.length(); i++) {
                c[s.charAt(i) - 'a']++;
                c[p.charAt(i) - 'a']--;
            }

            for (int i : c) {
                if (i != 0) {
                    return false;
                }
            }
            return true;
        }
    }


    public static class Solution2 {
        /**
         * Sliding Window
         */
        public List<Integer> findAnagrams(String s, String p) {
            List<Integer> result = new ArrayList();
            int[] hash = new int[26];
            for (char c : p.toCharArray()) {
                hash[c - 'a']++;
            }
            int start = 0;
            int end = 0;
            int count = p.length();
            while (end < s.length()) {
                if (hash[s.charAt(end) - 'a'] > 0) {
                    count--;
                }
                hash[s.charAt(end) - 'a']--;
                end++;

                if (count == 0) {
                    result.add(start);
                }

                if ((end - start) == p.length()) {
                    if (hash[s.charAt(start) - 'a'] >= 0) {
                        count++;
                    }
                    hash[s.charAt(start) - 'a']++;
                    start++;
                }
            }
            return result;
        }
    }
}