leetcode---combinations---dfs

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:    void dfs(int dep, int n, int k, vector< vector<int> > &ans, vector<int> &tmp)    {        if(tmp.size() == k)        {            ans.push_back(tmp);        }        for(int i=dep; i<=n; i++)        {            tmp.push_back(i);            dfs(i+1, n, k, ans,tmp);            tmp.pop_back();        }    }    vector<vector<int> > combine(int n, int k)     {        vector<int> tmp;        vector< vector<int> > ans;        dfs(1, n, k, ans, tmp);        return ans;    }};