[LC] 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.

Note:

• All numbers will be positive integers.
• The solution set must not contain duplicate combinations.

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]]

class Solution {
    public List<List<Integer>> combinationSum3(int k, int n) {
         List<List<Integer>> arrList = new ArrayList<>();
        List<Integer> list = new ArrayList<>();
        helper(arrList, list, 0, 1, k, n);
        return arrList;
    }
    
    private void helper(List<List<Integer>> arrList, List<Integer> list, int preSum, int index, int k, int n) {
        if (k == 0) {
            if (preSum == n) {
                arrList.add(new ArrayList<>(list));
            }
            return;
        }
        
        for (int i = index; i < 10; i++) {
            list.add(i);
            helper(arrList, list, preSum + i, i + 1, k - 1, n);
            list.remove(list.size() - 1);    
        }
    }
}