Symmetric Tree

Posted 2020-06-23 YuriFLAG

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

   　　 1
        /　\
  　　2   2
　　/   \  /  \
　  3   4 4   3

But the following is not:

    1
   / \
  2   2
   \   \
   3    3

递归 check左子树和右子树是否为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 public class Solution {
11     public boolean isSymmetric(TreeNode root) {
12         if (root == null || root.left == null && root.right == null) {
13             return true;
14         }
15         return helper(root.left, root.right);
16         
17     }
18     private boolean helper(TreeNode root1, TreeNode root2) {
19         if (root1 == null) {
20             return root2 == null;
21         }
22         if (root2 == null) {
23             return root1 == null;
24         }
25         if (root1.val != root2.val) {
26             return false;
27         }
28         return helper(root1.right, root2.left) && helper(root1.left, root2.right);
29     }
30 }