2.1.3.一串數列中任取n個數字, 總共有幾種組合

• example code

          class Solution {
           public:
            vector<vector<int>> subsets(vector<int>& nums) {
                vector<int> record;
                vector<vector<int>> res;
  
                for (int i = 0; i < nums.size(); i++){
                    helper(res, nums, record, i + 1 , 0);
            }
                //加入空list
                vector<int > temp ;
                res.push_back(temp);
                return res;
            }
            void helper(vector<vector<int>>& res, vector<int>&nums, vector<int>&record, int num, int start_point){
                //終止條件: 剩餘數量=0
                if (num == 0){
                    res.push_back(record);
                    return;
                }
                for (int i = start_point; i < nums.size(); i++){
                    record.push_back(nums[i]);
                    helper(res, nums,record, num - 1, i + 1);
                    record.pop_back();
                }
            }
      };