leetcode 39

DFS 基本能懂，需练习才能自主做

void combinationSumDFS(vector<vector<int>> &res,vector<int> &out,vector<int>& candidates,int target,int level)
    {
        if(target<0)
            return;
        else if(target==0)
        {
            res.push_back(out);
        }
        else{
            for(int i=level;i<candidates.size();i++)
            {
                out.push_back(candidates[i]);
                combinationSumDFS(res,out,candidates,target-candidates[i],i);
                out.pop_back();
            }
        }
    }
    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
        vector<vector<int>> res;
        vector<int> out;
        sort(candidates.begin(),candidates.end());
        combinationSumDFS(res,out,candidates,target,0);
        return res;
    }