Leetcode: Combination Sum III

ind 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]]

Use recursion, or DFS

One syntax to notice is line 3:

List<List<Integer>> res = new ArrayList<List<Integer>>();

List is abstract and can not be instantiated

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