Combinations（带for循环的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 {
private:
    vector<vector<int>> res;
    int myn;
    int num;
public:
    void tra(int loc,int start,vector<int> temp){
        if(loc==num+1)
        {
            res.push_back(temp);
            return;
        }
        for (int i=start;i<=myn;++i)
        {
            temp.push_back(i);
            tra(loc+1,i+1,temp);//不是start+1，而是i+1
            temp.erase(temp.end()-1);
        }
    }
    vector<vector<int>> combine(int n, int k) {
        myn=n;
        num=k;
        vector<int> temp;
        tra(1,1,temp);
        return res;
    }
};

 　　注释处是i+1，原因在于