101. Symmetric Tree

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 class Solution {
11     public boolean isSymmetric(TreeNode root) {
12         if(root == null){
13             return true;
14         }
15         
16         Deque<TreeNode> deque = new LinkedList<TreeNode>();
17         deque.addFirst(root.left);
18         deque.addLast(root.right);
19         
20         TreeNode preNode = null;
21         TreeNode postNode = null;
22         
23         while(!deque.isEmpty()){
24             preNode = deque.pollFirst();
25             postNode = deque.pollLast();
26             
27             if(preNode == null && postNode == null){
28                 continue;
29             }
30             
31             if(preNode == null || postNode == null){
32                 return false;
33             }
34             
35             if(preNode.val != postNode.val ){
36                 return false;
37             }else{
38                 deque.addFirst(preNode.right);
39                 deque.addFirst(preNode.left);
40                 
41                 deque.addLast(postNode.left);
42                 deque.addLast(postNode.right);
43             }
44         }
45         
46         return true;
47     }
48 }