Leetcode 39. Combination Sum solution using backtracking

39-Combination-Sum.cpp

class Solution {
private:
    vector<int> combination;
    vector<vector<int>> combinations;

public:
    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
        explore(candidates, 0, target);
        return combinations;
    }

    void explore(vector<int>& candidates, int start, int target){
        if(target == 0){
            combinations.push_back(combination); // yes! a valid solution found
            return;
        }    
        if(target < 0) return; // this is when we lose hope and backtrack :(

        for(int i = start; i < candidates.size(); i++){
            // explore all solutions using candidates[i] at least once
            combination.push_back(candidates[i]);
            explore(candidates, i, target - candidates[i]);
            
            // explore solutions that don't use candidates[i]
            combination.pop_back();
        }
    }
};