【leetcode】Combinations

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

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