Given a set of candidate numbers (C) (without duplicates) 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.
• 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>> results;  
        sort(candidates.begin(), candidates.end());  
        helper(results,vector
 
<int>{},candidates,target,0);  
        return results;  
    }  
    void helper(vector<vector<int>> &results, vector<int> result, vector<int>& c, int target, int index){  
        if(target == 0){  
            results.push_back(result);  
            return;  
        }  
        
 
for(int i = index; i < c.size() && target >= c[i]; ++i){  
            result.push_back(c[i]);  
            helper(results, result, c, target - c[i], i);  
            result.pop_back();  
        }  
    }  
};  

[leetcode]Backtracking-39. Combination Sum