Combination-Sum I

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]

Solution: Sort & Recursion.

 1 class Solution {
 2 public:
 3     vector<vector<int> > res;
 4     void combinationSum(vector<int> &candidates, int target, vector<int> &com, int start)
 5     {
 6         if(target == 0) {
 7             res.push_back(com);
 8             return;
 9         }
10         for(int i = start; i < candidates.size() && target >= candidates[i]; i++) {
11             com.push_back(candidates[i]);
12             combinationSum(candidates, target-candidates[i], com, i); // start = i;
13             com.pop_back();
14         }
15     }
16     
17     vector<vector<int> > combinationSum(vector<int> &candidates, int target) {
18         sort(candidates.begin(), candidates.end());
19         vector<int> com;
20         combinationSum(candidates, target, com, 0);
21         return res;
22     }
23 };