Subset

• Time : $O(nlogn + 2^n + 2^n * n)$
• Space : $O(n)$

void helper(vector<vector<int>>& res, vector<int>& A, vector<int>& now, int p) {
    if(p == A.size()) res.push_back(now);
    else {
        now.push_back(A[p]);
        helper(res,A,now,p+1);
        now.pop_back();
        helper(res,A,now,p+1);
    }
}
vector<vector<int>> Solution::subsets(vector<int> &A) {
    sort(begin(A), end(A));
    vector<vector<int>>res;
    helper(res, A, vector<int>() = {}, 0);
    sort(begin(res),end(res));
    return res;
}