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 [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

class Solution:
    def isSymmetric(self, root):
        if not root:
            return True
        return self.isMirrored(root.left, root.right)

    def isMirrored(self, left, right):

        if not left and not right:
            return True
        elif not left:
            return False
        elif not right:
            return False
        else:
            if left.val == right.val:
                return self.isMirrored(left.left, right.right) and \
                    self.isMirrored(left.right, right.left)
            else:
                return False