思路：先对数据进行排序（看评论给的测试数据好像都是有序数组了，但题目里没有给出这个条件），然后回溯加剪枝即可。

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