Combination Sum III

216. 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:

[[1,2,4]]

Example 2:

Input: k = 3, n = 9

Output:

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

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

Hope this helps,
Michael