Leetcode-Symmetric Tree

 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 boolean isSymmetric(TreeNode root) {
12         if (root==null)
13             return true;
14         
15         boolean res = isSymmetricRecur(root.left,root.right);
16         
17         return res;
18     }
19     
20     //Determine whether the tree "left" and the tree "right" is symmetric.
21     public boolean isSymmetricRecur(TreeNode left, TreeNode right){
22         //One is null, the other is not, then false;
23         if ((left==null && right!=null)||(left!=null&&right==null))
24             return false;
25             
26         //Both are empty, then true;
27         if (left==null&&right==null)
28             return true;
29         
30         //Both are not empty, but val is not the same, then false.
31         if (left.val!=right.val)
32             return false;
33         
34         //Both of them are leaf nodes
35         if (left.left==null&&left.right==null&&right.left==null&&right.right==null)
36             if (left.val==right.val)
37                 return true;
38             else 
39                 return false;
40         
41         boolean leftSym = isSymmetricRecur(left.left,right.right);
42         if (!leftSym)
43             return false;
44             
45         boolean rightSym = isSymmetricRecur(left.right,right.left);
46         if (!rightSym)
47             return false;
48             
49             
50         return true;
51     }
52 }