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FindKthNumber.java
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FindKthNumber.java
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package com.thealgorithms.maths;
import java.util.Random;
/**
* Use a quicksort-based approach to identify the k-th largest or k-th max element within the provided array.
*/
public final class FindKthNumber {
private FindKthNumber() {
}
private static final Random RANDOM = new Random();
public static int findKthMax(int[] array, int k) {
if (k <= 0 || k > array.length) {
throw new IllegalArgumentException("k must be between 1 and the size of the array");
}
// Convert k-th largest to index for QuickSelect
return quickSelect(array, 0, array.length - 1, array.length - k);
}
private static int quickSelect(int[] array, int left, int right, int kSmallest) {
if (left == right) {
return array[left];
}
// Randomly select a pivot index
int pivotIndex = left + RANDOM.nextInt(right - left + 1);
pivotIndex = partition(array, left, right, pivotIndex);
if (kSmallest == pivotIndex) {
return array[kSmallest];
} else if (kSmallest < pivotIndex) {
return quickSelect(array, left, pivotIndex - 1, kSmallest);
} else {
return quickSelect(array, pivotIndex + 1, right, kSmallest);
}
}
private static int partition(int[] array, int left, int right, int pivotIndex) {
int pivotValue = array[pivotIndex];
// Move pivot to end
swap(array, pivotIndex, right);
int storeIndex = left;
// Move all smaller elements to the left
for (int i = left; i < right; i++) {
if (array[i] < pivotValue) {
swap(array, storeIndex, i);
storeIndex++;
}
}
// Move pivot to its final place
swap(array, storeIndex, right);
return storeIndex;
}
private static void swap(int[] array, int i, int j) {
int temp = array[i];
array[i] = array[j];
array[j] = temp;
}
}