216. Combination Sum III java solutions

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]]

Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.

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 1 public class Solution {
 2     public List<List<Integer>> combinationSum3(int k, int n) {
 3         ArrayList<List<Integer>> ans = new ArrayList<List<Integer>>();
 4         ArrayList<Integer> tmp = new ArrayList<Integer>();
 5         if(k < 1 || n < 1) return ans;
 6         combinationDFS(ans, tmp, k, n, 1, 0);
 7         return ans;
 8     }
 9
10     public void combinationDFS(ArrayList<List<Integer>> ans, ArrayList<Integer> tmp, int k, int n, int level, int sum){
11         if(sum == n && k == 0){
12             ans.add(new ArrayList(tmp));
13             return;
14         }else if(sum > n || k < 0){
15             return;
16         }
17         for(int i = level; i<=9; i++){
18             tmp.add(i);
19             combinationDFS(ans,tmp,k-1,n,i+1,sum+i);
20             tmp.remove(tmp.size()-1);
21         }
22     }
23 }