sort by digit sum

#include <bits/stdc++.h>

using namespace std;

int digitSum(int x) {
    if (x < 10) {
        return x;
    } else {
        return digitSum(x / 10) + x % 10;
    }
}

bool cmp(int x, int y) {
    return (digitSum(x) < digitSum(y) || digitSum(x) == digitSum(y) && x < y);
}

int main() {
    std::ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    int n;
    cin >> n;
    int a[n];
    for (int i = 0; i < n; ++i) {
        cin >> a[i];
    }

    // [0; n)
    sort(a + 0, a + n, &cmp);

    for (int i = 0; i < n; ++i) {
        cout << a[i] << " ";
    }

    return 0;
}