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

Java

public class Solution {
    public boolean isSymmetric(TreeNode root) {
        if(root == null) {
            return true;
        }
        return isSymmetric(root.left, root.right);
    }

    public boolean isSymmetric(TreeNode left, TreeNode right) {
        if(left == null && right == null) {
            return true;
        } else if(left != null && right != null && left.val == right.val) {
            return isSymmetric(left.left, right.right) && isSymmetric(left.right, right.left);
        } else {
            return false;
        }
    }
}