[Leetcode] 101. 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

Note:

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

/**
* Definition for binary tree
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public boolean isSymmetric(TreeNode root) {
if(root == null) return true;
return helper(root.left, root.right);
}
private boolean helper(TreeNode left, TreeNode right){
if(left == null && right == null) return true;
if(left == null || right == null) return false;
if(left.val != right.val) return false;
return helper(left.left, right.right) && helper(left.right, right.left);
}
}