CombinationSumIII.java

/*
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]]
*/

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