Leetcode: 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.

Ensure that numbers within the set are sorted in ascending order.

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.

Understand the problem:
A very classic back tracking problem.

Code (Java):

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public class Solution {     public List<List<Integer>> combinationSum3(int k, int n) {         List<List<Integer>> result = new ArrayList<List<Integer>>();                   if (k <= 0 || n <= 0) {             return result;         }                   List<Integer> curr = new ArrayList<Integer>();         combinationSum3Helper(1, n, 0, k, 0, curr, result);                   return result;     }           private void combinationSum3Helper(int start, int n, int count, int k, int curSum, List<Integer> curr,                                        List<List<Integer>> result) {         if (count > k) {             return;         }                   if (curSum == n && count == k) {             result.add(new ArrayList<Integer>(curr));             return;         }                   for (int i = start; i <= 9; i++) {             if (curSum + i > n) {                 break;             }             curr.add(i);             combinationSum3Helper(i + 1, n, count + 1, k, curSum + i, curr, result);             curr.remove(curr.size() - 1);         }     } }