Combination Sum -- Leetcode

Question:

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] 

Answer:
DFS method, using backtrack!
Note: each element can be used for more than one time!

class Solution {
public:
    vector<vector<int> > combinationSum(vector<int> &candidates, int target) {
        sort(candidates.begin(),candidates.end());
        vector<int> subres;
        vector<vector<int>> res;
        conbinationSumDFS(subres,res,candidates,0,target);
        return res;
    }
   
    void conbinationSumDFS(vector<int> &subres, vector<vector<int>> &res, vector<int> &candidates, int start, int sum){
        if(sum==0){
            res.push_back(subres);
            return;
        }
       
        for(int i= start; i< candidates.size();++i){
            if(candidates[i] <= sum){
                subres.push_back(candidates[i]);
                conbinationSumDFS(subres,res,candidates,i,sum-candidates[i]);   //Note the start index is i !!!
                subres.pop_back();
            }
            else
                break;
        }
        return;
    }
};