101. Symmetric Tree

Posted 2021-03-09 hyxsolitude

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

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

    1
   /   2   2
 /  / 3  4 4  3

 

But the following [1,2,2,null,3,null,3] is not:

    1
   /   2   2
         3    3

 

Note:
Bonus points if you could solve it both recursively and iteratively.

 

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