LeetCode: Combinations

class Solution {
public:

    vector < vector <int> > ans;
    int mark[20 + 10];
    int N, K;

    void f(int k, int prev){
        if(k == K){
            vector <int> cur;
            for(int i=1;i<=N;i++)
                if(mark[i]) cur.push_back(i);
            ans.push_back(cur);
        }
        for(int i=prev+1;i<=N;i++){
            if(!mark[i]){
                mark[i] = true;
                f(k + 1, i);
                mark[i] = false;
            }
        }
    }

    vector<vector<int>> combine(int n, int k) {
        N = n, K = k;
        f(0, 0);
        return ans;
    }
};