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] 

Analyse: First sort the vector, then recursively invoke the dfs function.

Runtime: 16ms.

class Solution {
public:
    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
        sort(candidates.begin(), candidates.end());
        vector<vector<int> >result;
        vector<int> temp;
        
        dfs(temp, candidates, target, 0, result);
        return result;
    }
    
    void dfs(vector<int> &temp, vector<int> &candidates, int diff, int start, vector<vector<int> > &result){
        if(diff == 0){
            result.push_back(temp);
            return ;
        }
        
        for(int i = start; i < candidates.size(); i++){
            if(diff < candidates[i]) return;
            
            temp.push_back(candidates[i]);
            dfs(temp, candidates, diff - candidates[i], i, result);
            temp.pop_back();
        }
    }
};