Combinations

• Time : $O({}_A \mathrm{ C }_B)$
• Space : $O(B)$

void helper(vector<vector<int>>& res, int now, int n, int rem, vector<int>& A) {
    if(rem == 0) res.push_back(A);
    else {
        A.push_back(now);
        helper(res, now + 1, n, rem - 1, A);
        A.pop_back();
        if(n - now >= rem) helper(res,now+1,n,rem,A);
    }
}
vector<vector<int>> Solution::combine(int A, int B) {
    if(A < B) return {};
    vector<vector<int>> res;
    helper(res, 1, A, B, vector<int>() = {});
    return res;
}