216. Combination Sum III

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Problem

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]]

Code

class Solution {
    public List<List<Integer>> combinationSum3(int k, int n) {
        List<List<Integer>> res = new ArrayList<>();
        helper(res, new ArrayList<>(), k, n, 1);
        return res;
    }

    private void helper(List<List<Integer>> res, List<Integer> temp, int k, int n, int start){
        if(k == 0 && n == 0){
            res.add(new ArrayList<>(temp));
            return;
        }

        for(int i = start; i <= 9; i++){
            temp.add(i);
            helper(res, temp, k - 1, n - i, i + 1);
            temp.remove(temp.size() - 1);
        }
    }
}