combine题解

class Solution {public:    vector<vector<int> > combine(int n, int k) {        vector<vector<int> > res;        vector<int> out;        combineDFS(n, k, 1, out, res);        return res;    }    void combineDFS(int n, int k, int level, vector<int> &out, vector<vector<int> > &res) {        if (out.size() == k) res.push_back(out);        else {            for (int i = level; i <= n; ++i) {                out.push_back(i);                combineDFS(n, k, i + 1, out, res);                out.pop_back();            }        }    }};