Combination sum

思路：此题很简单，只需用DFS每次添加一个元素即可。 
1 class Solution {
public:
    
    void DFS(vector<int> candidates, int target, vector<vector<int> > &result,int k,vector<int> a){
        
        if (k == target/min(candidates.front(),candidates.back())+1) return;
        if (a.size() != 0){
            
            int sum = 0;
            for (int i=0; i<a.size(); i++) sum += a[i];
            if (sum == target) result.push_back(a);
        }
        
      
            
        k = k+1;
        for (int i=0;i<candidates.size();i++){
                
            a.push_back(candidates[i]);
            DFS(candidates,target,result,k,a);
            a.pop_back();
        }
    }
    
    vector<vector<int> > combinationSum(vector<int> &candidates, int target) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<vector<int> > result;
        if (candidates.size() == 0) return result;
        
        int sum =0;
        int k = 0;
        vector <int> a;
        DFS(candidates,target,result,k,a);
        
        map<vector<int>, int> map1;
        if (result.size()>0){
            for (int i = 0; i<result.size(); i++){
                sort(result[i].begin(),result[i].end());
                map1[result[i]]= i;
                
            }
            
            result.clear();
            map<vector<int>, int>::iterator it;
            for (it=map1.begin(); it!=map1.end();it++)
                result.push_back(it->first);
            
            
        }
        return result;
    }
    
};