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]]

 1 public class Solution {
 2     public IList<IList<int>> CombinationSum3(int k, int n) {
 3         var results = new List<IList<int>>();
 4         DFS(k, n, 1, 0, 0, new List<int>(), results);
 5         return results;
 6     }
 7     
 8     private void DFS(int k, int
 
 n, int last, int cur, int sum, IList<int> result, IList<IList<int>> results)
 9     {
10         if (cur >= k || sum >= n)
11         {
12             if (cur == k && sum == n)
13             {
14                 results.Add(new List<int>(result));                
 
15             }
16             
17             return;
18         }
19         
20         for (int i = last; i <= 9; i++)
21         {
22             result.Add(i);
23             DFS(k, n, i + 1, cur + 1, sum + i, result, results);
24             result.RemoveAt(result.Count - 1);
25         }
26     }
27 }

Leetcode 216: Combination Sum III