leetcode_39.cpp

class Solution {
public:
     vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
        vector<vector<int>> result;
        vector<int> combination;
        backtrack(candidates, target, 0, combination, result);
        return result;
    }
private:
     void backtrack(vector<int>& candidates, int remain, int start, 
                   vector<int>& combination, vector<vector<int>>& result){
        if(remain == 0) { 
            result.push_back(combination);
            return;
        }
        if(remain < 0) {
            return;
        }
        for(int i = start; i < candidates.size(); i++){
            combination.push_back(candidates[i]);
            backtrack(candidates, remain - candidates[i], i, combination, result);
            combination.pop_back();
        }
    }
};