LeetCode - Symmetric Tree

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

Solution:

 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 check(TreeNode r1, TreeNode r2){

12         if(r1 == null && r2 == null) 

13             return true;

14         if(r1 == null || r2 == null)

15             return false;

16         if(r1.val != r2.val)

17             return false;

18         return check(r1.left, r2.right) && check(r1.right, r2.left);

19     }

20     public boolean isSymmetric(TreeNode root) {

21         // Start typing your Java solution below

22         // DO NOT write main() function

23         if(root == null) return true;

24         return check(root.left, root.right);

25     }

26 }