qsort.c

void qsort(int a[], int l, int r) {
    int first, last, key;
    first = l; last = r;
    key = a[last];
    while (first < last) {
        while (first<last && a[first]<=key) first++;
        a[last] = a[first];
        while (first<last && a[last]>=key) last--;
        a[first] = a[last];
    }
    a[first] = key;
    if (l < first-1) qsort(a, l, first-1);
    if (r > first+1) qsort(a, first+1, r);
}