void quick_sort(int* a, int low, int high){ if (low >= high) {
    return;
} int first = low; int last = high; int key = a[first]; while (first < last) {
    while (first < last && a[last] >= key) {
        --last;
    }
    a[first] = a[last];
 
    while (first < last && a[first] <= key) {
        ++first;
    }
    a[last] = a[first];
}
a[first] = key;
quick_sort(a, low, first-1);
quick_sort(a, first+1, high);
}