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):

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);
        }
    }
}