[LeetCode]Combinations

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],
]

Have you been asked this question in an interview? Yes

class Solution {
private:
    vector<vector<int> > res;
    vector<int> ans;
public:
    void DFS(int n,int k,int start,int dep)
    {
        if(dep==k)
        {
            res.push_back(ans);
            return;
        }
        for(int i=start;i<n-k+2;i++)
        {
            ans.push_back(i+dep);
            DFS(n,k,i,dep+1);
            ans.pop_back();
        }
    }
    vector<vector<int> > combine(int n, int k) {
        res.clear();
        ans.clear();
        DFS(n,k,1,0);
        return res;
    }
};