216. Combination Sum III

# Medium

Key idea: backtracking

class Solution {
public:
    vector<vector<int>> combinationSum3(int k, int n) {
        vector<int> subComb;
        vector<vector<int>> comb;
        helper(comb, subComb, k, n, 1);
        return comb;
    }
    
    void helper(vector<vector<int>> &comb, vector<int> &subComb, int k, int n, int i) {
        // stop condition
        if(subComb.size() == k) {
            if(n == 0) comb.push_back(subComb);
            return; 
        }
        
        // regular case
        for(int j = i; j <= 9; j ++) {
            subComb.push_back(j);
            helper(comb, subComb, k, n-j, j+1);
            subComb.pop_back();
        }
    }
};

Still confuse whether vector<int>subComb should use reference or original list? But using reference makes this program faster, beat 100%.