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. [[1,2,4]]

Example 2:

Input: k = 3, n = 9

Output:

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

Solution:

  1. public class Solution {
  2.   public List<List<Integer>> combinationSum3(int k, int n) {
  3.     List<List<Integer>> res = new ArrayList<List<Integer>>();
  4.     helper(k, n, 1, new ArrayList<Integer>(), res);
  5.     return res;
  6.   }
  8.   private void helper(int k, int n, int index, List<Integer> sol, List<List<Integer>> res) {
  9.     if (n == 0 && k == sol.size()) {
  10.       res.add(new ArrayList<Integer>(sol));
  11.       return;
  12.     }
  14.     if (index > 9 || n < 0) {
  15.       return;
  16.     }
  18.     for (int i = index; i <= 9; i++) {
  19.       sol.add(i);
  20.       helper(k, n - i, i + 1, sol, res);
  21.       sol.remove(sol.size() - 1);
  22.     }
  23.   }
  24. }