n 개 중 m 개 고르는 조합 (all possible combinations)

void pick(int n, vector<int> &picked, int remain) {
    if (remain == 0) {
        //do_something_with(picked);
        return;
    }

    int mini = picked.empty() ? 0 : picked.back() + 1;
    for (int i = mini; i < n; i++) {
        picked.push_back(i);
        pick(n, picked, remain - 1);
        picked.pop_back();
    }
}

uint32 next(uint32 v) {
    uint32 w;
    uint32 t = v | (v - 1);
    w = (t + 1) | (((~t & -~t) - 1) >> (__builtin_ctz(v) + 1));
    return w;
}

// n <= 31
void pick(int n, int m) {
    uint32 i = (1 << m) - 1;
    uint32 maxi = (1 << n) - 1;
    while (maxi >= i) {
        //do_something_with(i);
        i = next(i);
    }
}