（效率低下）77. Combinations C++回溯法

沿用78题的思路

class Solution {
public:
    void backTrack(vector<int> ans, vector<int> nums, vector<vector<int>>& res, int times,int k)
    {
        if(ans.size() == k)
        {
            res.push_back(ans);
        }
        else
        {
            for(int i = times; i<nums.size(); i++)
            {
                ans.push_back(nums[i]);
                backTrack(ans,nums,res,i+1,k);
                ans.pop_back();
            }
        }
    }
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> res;
        vector<int> ans;
        vector<int> nums;
        for(int i=1;i<=n;i++)
            nums.push_back(i);
        int bgi = 0;
        backTrack(ans,nums,res,bgi,k);
        return res;
    }
};