leetcode 77: Combinations

Use DFS

class Solution {public:    vector<vector<int>> combine(int n, int k) {        vector<int> set;        vector<vector<int> > res;        helper(0,n,k,set,res);        return res;    }    void helper(int last, int n, int k, vector<int> set, vector<vector<int> >& res)    {        if(k==0)        {            res.push_back(set);            return;        }        for(int i=last+1;i<=n-k+1;i++)        {            set.push_back(i);            helper(i,n,k-1,set,res);            set.pop_back();        }    }};