quicksort(with call tail optimization)

#include<bits/stdc++.h>
using namespace std;

int partition(int a[], int l, int r) {
    int pivot = a[r];
    int i = l - 1;
    for (int j = l; j < r; j++) {
        if (a[j] <= pivot) {
            i++;
            swap(a[i], a[j]);
        }
    }
    swap(a[i + 1], a[r]);
    return i + 1;
}

void quicksort(int a[], int l, int r) {
    while (l < r) {
        int pivot = partition(a, l, r);
        if (pivot - l < r - pivot) {
            quicksort(a, l, pivot - 1);
            l = pivot + 1;
        } else {
            quicksort(a, pivot + 1, r);
            r = pivot - 1;
        }
    }
}

int a[] = {20, 3, 2, 1, 4, 0, 9, -1};
int main () {
    quicksort(a, 1, 7);
    for (int i = 1; i <= 7; i++)
        std::cerr << a[i] << '\n';
    return 0;
}