077.Combinations

Posted on | In

Solution 1: accepted 28ms

DFS. 

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        List<Integer> list = new ArrayList<>();
        if (n == 0) {
            return result;
        }
          
        comboHelper(result, list, n, k);
        return result;
    }
    
    private void comboHelper(List<List<Integer>> result,
                            List<Integer> list,
                            int n,
                            int k) {
        if (k == 0) {
            result.add(new ArrayList<>(list));
        } else {
            for (int i = n; i > 0; i--) {
                list.add(i);
                comboHelper(result, list, i - 1, k - 1);
                list.remove(list.size() - 1);
            }
        }
    }
}