Combination Sum

Given a set of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.

The same repeated number may be chosen from C unlimited number of times.

Note:

• All numbers (including target) will be positive integers.
• Elements in a combination (a1, a2, … , ak) must be in non-descending order. (ie, a1 ≤ a2 ≤ … ≤ ak).
• The solution set must not contain duplicate combinations.

For example, given candidate set 2,3,6,7 and target 7, 
A solution set is: 
[7] 
[2, 2, 3] 

class Solution {
public:
    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
        vector<vector<int>>res;
        vector<int>tmp;
        sort(candidates.begin(),candidates.end());
        helper(candidates,tmp,res,target,0);
        return res;
    }
    void helper(vector<int>&candidates,vector<int>tmp,vector<vector<int>>& res,int target,int start){
        if(start>=candidates.size()) return;
        if(target==0){
            res.push_back(tmp);
            return;
        }
        else if(target<0) return ;
        else{
            helper(candidates,tmp,res,target,start+1);
            tmp.push_back(candidates[start]);
            helper(candidates,tmp,res,target-candidates[start],start);
        }
    }
};