77. Combinations

 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 }