input: from typing import List
  
  class TreeNode:
      def __init__(self, x):
          self.val = x
          self.left = None
          self.right = None
  
  def are_similar(root1, root2):
      """ Compare two binary trees to check if they are similar. Two trees are considered similar if they have the same structure and the same values at corresponding nodes.
      >>> root1 = TreeNode(1)
      >>> root1.left = TreeNode(2)
      >>> root1.right = TreeNode(3)
      >>> root2 = TreeNode(1)
      >>> root2.left = TreeNode(2)
      >>> root2.right = TreeNode(3)
      >>> are_similar(root1, root2)
      True
      >>> root1 = TreeNode(1)
      >>> root1.left = TreeNode(2)
      >>> root2 = TreeNode(1)
      >>> root2.right = TreeNode(2)
      >>> are_similar(root1, root2)
      False
      """
output:     if not root1 and not root2:
          return True
      if not root1 or not root2:
          return False
      if root1.val != root2.val:
          return False
      return are_similar(root1.left, root2.left) and are_similar(root1.right, root2.right)