39. Combination Sum

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

这个题，回溯法，很重要，那个pop_back()好好理解