本題還是基本的回溯法。就是回溯函式的引數選擇上要花點心思！

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