Example: Quick Sort using Recursion in C

// Quick Sort using Recursion
#include <stdio.h>

// Function to swap two elements
void swap(int* a, int* b) {
    int t = *a;
    *a = *b;
    *b = t;
}

/*
 * This function takes the last element as pivot, places
 * the pivot element at its correct position in sorted
 * array, and places all smaller (than pivot) to left
 * of pivot and all greater elements to right of pivot.
 */
int partition(int arr[], int low, int high) {
    // Step 1: Choose the pivot (last element in this implementation)
    int pivot = arr[high];
    // Index of smaller element and indicates the right position of the pivot found so far
    int i = (low - 1);

    // Step 2: Iterate through the array from low to high-1
    for (int j = low; j <= high - 1; j++) {
        // If current element is smaller than the pivot
        if (arr[j] < pivot) {
            i++; // Increment index of smaller element
            swap(&arr[i], &arr[j]); // Swap current element with element at i
        }
    }
    // Step 3: Place the pivot element at its correct sorted position
    swap(&arr[i + 1], &arr[high]);
    return (i + 1); // Return the partitioning index
}

/*
 * The main function that implements QuickSort
 * arr[] --> Array to be sorted,
 * low   --> Starting index,
 * high  --> Ending index
 */
void quickSort(int arr[], int low, int high) {
    // Step 1: Base case for recursion
    if (low < high) {
        // Step 2: pi is partitioning index, arr[pi] is now at right place
        int pi = partition(arr, low, high);

        // Step 3: Recursively sort elements before partition and after partition
        quickSort(arr, low, pi - 1);  // Sort left sub-array
        quickSort(arr, pi + 1, high); // Sort right sub-array
    }
}

// Function to print an array
void printArray(int arr[], int size) {
    for (int i = 0; i < size; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");
}

int main() {
    int arr[] = {10, 7, 8, 9, 1, 5};
    int n = sizeof(arr) / sizeof(arr[0]);

    printf("Original array: ");
    printArray(arr, n);

    quickSort(arr, 0, n - 1);

    printf("Sorted array: ");
    printArray(arr, n);

    return 0;
}