題目地址：https://leetcode-cn.com/problems/combination-sum/
思路：dfs，排序剪枝
AC程式碼：

class Solution {
public:
    vector<vector<int>>ans;
    vector<int>res;
    void solve(vector<int>& candidates, int target,int k){
        if(target==0){
            ans.push_back(res);
            return;
        }
        int n = candidates.size();
        for(int i=k;i<n;i++){
            if(target>=candidates[i]){
                res.push_back(candidates[i]);
                solve(candidates,target-candidates[i],i);
                res.pop_back();
            }else
                return;
        }
    }
    
    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
        sort(candidates.begin(),candidates.end());
        solve(candidates,target,0);
        return ans;
    }
};