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Merged
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Mar 12, 2022
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find kth Element in an Array #2968

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find kth Element in an Array
jsrdxzw Mar 4, 2022
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Merge branch 'master' into master
siriak Mar 12, 2022
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73 changes: 73 additions & 0 deletions src/main/java/com/thealgorithms/maths/FindKthNumber.java
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package com.thealgorithms.maths;

import java.util.Arrays;
import java.util.Random;

/**
* use quick sort algorithm to get kth largest or kth smallest element in given array
*/
public class FindKthNumber {
private static final Random random = new Random();

public static void main(String[] args) {
/* generate array with random size and random elements */
int[] nums = generateArray(100);

/* get 3th largest element */
int kth = 3;
int kthMaxIndex = nums.length - kth;
int targetMax = findKthMax(nums, kthMaxIndex);

/* get 3th smallest element */
int kthMinIndex = kth - 1;
int targetMin = findKthMax(nums, kthMinIndex);

Arrays.sort(nums);
assert nums[kthMaxIndex] == targetMax;
assert nums[kthMinIndex] == targetMin;
}

private static int[] generateArray(int capacity) {
int size = random.nextInt(capacity) + 1;
int[] array = new int[size];

for (int i = 0; i < size; i++) {
array[i] = random.nextInt() % 100;
}
return array;
}

private static int findKthMax(int[] nums, int k) {
int start = 0, end = nums.length;
while (start < end) {
int pivot = partition(nums, start, end);
if (k == pivot) {
return nums[pivot];
} else if (k > pivot) {
start = pivot + 1;
} else {
end = pivot;
}
}
return -1;
}

private static int partition(int[] nums, int start, int end) {
int pivot = nums[start];
int j = start;
for (int i = start + 1; i < end; i++) {
if (nums[i] < pivot) {
j++;
swap(nums, i, j);
}
}
swap(nums, start, j);
return j;
}

private static void swap(int[] nums, int a, int b) {
int tmp = nums[a];
nums[a] = nums[b];
nums[b] = tmp;
}
}