39. Combination Sum

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