216_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<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 n, int pos) {
        if(k == 0 || n <= 0) {
            if(k == 0 && n == 0) {
                res.add(new ArrayList<Integer>(list));
            }
            return;
        }
        for(int i=pos; i<=9; i++) {
            list.add(i);
            helper(res, list, k - 1, n - i, i + 1);
            list.remove(list.size() - 1);
        }
    }
}