101. 对称二叉树

 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     private boolean ismirror(TreeNode left, TreeNode right){
12         if(left==null&&right==null){
13             return true;
14         }else if(left==null||right==null){
15             return false;
16         }else if(left.val!=right.val){
17             return false;
18         }else{
19             return ismirror(left.left,right.right)&&ismirror(left.right,right.left);
20         }
21     }
22     public boolean isSymmetric(TreeNode root) {
23         if(root == null)return true;
24         return ismirror(root.left,root.right);
25     }
26 }