Combination Sum III

Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.


Example 1:

Input: k = 3, n = 7

Output:

[[1,2,4]]

Example 2:

Input: k = 3, n = 9

Output:

[[1,2,6], [1,3,5], [2,3,4]]

class Solution {
public:
    vector<vector<int>> combinationSum3(int k, int n) {
        vector<vector<int>> res;
        vector<int> sol;

        dfs(k, n, sol, res);
        return res;
    }
private:
    void dfs(int level, int sum, vector<int> sol, vector<vector<int>> &ret){

        if(sol.size() == level && sum == 0)  {
            ret.push_back(sol);
            return;
        }

        if(sol.size() < level){
            for(int i = sol.empty() ? 1: sol.back() + 1; i <= 9; i++){
                if(sum - i < 0)   break;
                sol.push_back(i);
                dfs(level, sum - i, sol, ret);
                sol.pop_back();
            }
        }
    }
};