1 class Solution {
2 List<List<Integer>> res = new ArrayList<>();
3 public List<List<Integer>> combine(int n, int k) {
4 if(k == 0) return null;
5 int[] nums = new int[n];
6 for(int i = 0; i < n; i++) {
7 nums[i] = i+1;
8 }
9 backtrack(nums, k, new ArrayList<>(), 0);
10 return res;
11
12 }
13
14 public void backtrack(int[] nums, int k, List<Integer> list, int position) {
15 if(list.size() == k) {
16 res.add(new ArrayList<>(list));
17 return;
18 }
19 for(int i = position; i < nums.length; i++) {
20 list.add(nums[i]);
21 backtrack(nums, k, list, i+1);
22 list.remove(list.size() - 1);
23 }
24 }
25 }
