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quick_select.c
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quick_select.c
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// Path: C\Searching\quick_select.c
// C program to kth smallest element using quickSelect Algorithm.
// Time-Complexity: O(N^2), where N is the size of array.
#include <stdio.h>
#include <stdlib.h>
// Function to swap two elements in the array
void swap(int *a, int *b) {
int temp = *a;
*a = *b;
*b = temp;
}
// Partition function to rearrange elements around the pivot
int partition(int arr[], int left, int right, int pivotIndex) {
int pivotValue = arr[pivotIndex];
swap(&arr[pivotIndex], &arr[right]); // Move pivot to the end
int storeIndex = left;
for (int i = left; i < right; i++) {
if (arr[i] < pivotValue) {
swap(&arr[i], &arr[storeIndex]);
storeIndex++;
}
}
swap(&arr[storeIndex], &arr[right]); // Move pivot to its final place
return storeIndex;
}
// Quickselect function
int quickSelect(int arr[], int left, int right, int k) {
if (left == right)
return arr[left];
int pivotIndex = left + (rand() % (right - left + 1));
pivotIndex = partition(arr, left, right, pivotIndex);
if (k == pivotIndex)
return arr[k];
else if (k < pivotIndex)
return quickSelect(arr, left, pivotIndex - 1, k);
else
return quickSelect(arr, pivotIndex + 1, right, k);
}
int main() {
int arr[] = {3, 8, 2, 5, 1, 4, 7, 6};
int n = sizeof(arr) / sizeof(arr[0]);
int k = 4; // Find the 4th smallest element
int result = quickSelect(arr, 0, n - 1, k - 1);
printf("The %d-th smallest element is: %d\n", k, result);
return 0;
}