Binary Tree Level Order Traversal II

 1 /**
 2  * Definition for binary tree
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public List<List<Integer>> levelOrderBottom(TreeNode root) {
12         LinkedList<List<Integer>> rt = new  LinkedList<List<Integer>>();
13         
14         if(root == null) return rt;
15         
16         final TreeNode END = new TreeNode(0);
17         
18         Deque<TreeNode> queue = new LinkedList<TreeNode>();
19         
20         queue.add(root);
21         queue.add(END);
22         
23         ArrayList<Integer> level = new ArrayList<Integer>();
24         
25         while(!queue.isEmpty()){
26             
27             TreeNode node = queue.poll();
28             
29             if(node == END){
30                 rt.push(new ArrayList<Integer>(level)); // copy
31                 level.clear();
32                 
33                 if(!queue.isEmpty()) queue.add(END);
34                 
35             }else{
36                 
37                 level.add(node.val);
38                                 
39                 if(node.left != null)  queue.addLast(node.left);
40                 if(node.right != null) queue.addLast(node.right);
41             }
42         }
43         
44         return rt;
45     }
46 }