代码仍然出自代码随想录，此处用于记录。主要是回溯方法的应用和剪枝的具体处理。

#include <bits/stdc++.h>

using namespace std;

class Solution {
public:

        vector<vector<int>>res;
        vector<int>path;
        void backtracking(vector<int>&candidates,int target,int sum, int startIndex){
            if (sum == target){
                res.push_back(path);
                return;
            }
            //sum+candidates[i] + sum > target,终止遍历（此处进行剪枝)
            for (int i = startIndex; i < candidates.size() && sum + candidates[i] <= target; i++){
                sum += candidates[i];
                path.push_back(candidates[i]);
                backtracking(candidates,target,sum,i);//此处是因为可以使用多次
                sum -= candidates[i];
                path.pop_back();
            }
        }

      vector<vector<int>> combinationSum(vector<int>& candidates, int target)
      {
          sort(candidates.begin(),candidates.end());//事先对数据进行排序
          backtracking(candidates,target,0,0);
          return res;

      }


};

int main()
{
    vector<int>candidates = {2,3,6,7};
    int target = 7;
    vector<vector<int>> res = Solution().combinationSum(candidates,target);
    for (int i = 0; i < res.size(); i++){
        for (int j = 0; j < res[i].size(); j++){
            cout << " " << res[i][j];
        }
        cout << endl;
    }
    return 0;
}