leetcode 216. Combination Sum III (DFS)

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.

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]]

dfs，把可能的情况列出来

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