普通のクイックソートなり.

quick_sort.c

#include <stdio.h>

void print_array(int * array, int size) {
    int i = 0;
    printf("{ ");
    while (1) {
        printf("%d", array[i]);
        if (++i >= size) {
            printf("}\n");
            break;
        } else {
            printf(",");
        }
    }
}

int pivot(int *array, int begin, int end) {
    int i = begin + 1;
    while (i <= end && array[begin] == array[i]) i++;
    if (i > end)
        return -1;
    if (array[begin] >= array[i])
        return begin;
    return i;
}

int partition(int *array, int begin, int end, int pivot_value) {
    int left = begin, right = end;
    while (left <= right) {
        while (left <= end && array[left] < pivot_value) left++;
        while (right >= begin && array[right] >= pivot_value) right--;
        if (left > right) break;
        else {
            int temp = array[left];
            array[left] = array[right];
            array[right] = temp;
            left++;  right--;
        }
    }
    return left;
}

void quick_sort(int *array, int begin, int end) {
    if (begin == end) return;
    int pivot_index = pivot(array, begin, end);
    if (pivot_index != -1) {
        int partition_point = partition(array, begin, end, array[pivot_index]);
        quick_sort(array, begin, partition_point -1);
        quick_sort(array, partition_point, end);
    }
}

int main(void) {
    int i;
    int tests[4][5] = {
        { 1, 2, 3, 4, 5 },
        { 2, 3, 4, 5, 1 },
        { 4, 5, 3, 2, 3 },
        { 5, 4, 3, 1, 1 }
    };
    for (i = 0; i < 4; i++) {
        quick_sort(tests[i], 0, 4);
        print_array(tests[i], 5);
    }
    return 0;
}