Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.For example,If n = 4 and k = 2, a solution is:

[  [2,4],  [3,4],  [2,3],  [1,2],  [1,3],  [1,4],]

class Solution {public:    vector<vector<int> > combine(int n, int k) {        vector<vector<int>> result;        if (n == 0) return result;        vector<int> path;        dfs(n, k, 1, 0, path, result);        return result;    }        void dfs(int n, int k, int start, int cur, vector<int>& path, vector<vector<int>>& result) {        if (cur == k) {            result.push_back(path);        }            for (int i = start; i <= n; ++i) {            path.push_back(i);            dfs(n, k, i + 1, cur + 1, path, result);            path.pop_back();        }    }};