input: from typing import Optional
  
  class TreeNode:
      def __init__(self, x):
          self.val = x
          self.left = None
          self.right = None
  
  def are_similar(root1: Optional[TreeNode], root2: Optional[TreeNode]) -> bool:
      """
      Check if two binary trees are similar. Two trees are similar if they have the same structure
      and the same values at corresponding nodes.
      
      >>> tree1 = TreeNode(1)
      >>> tree1.left = TreeNode(2)
      >>> tree1.right = TreeNode(3)
      >>> tree2 = TreeNode(1)
      >>> tree2.left = TreeNode(2)
      >>> tree2.right = TreeNode(3)
      >>> are_similar(tree1, tree2)
      True
      
      >>> tree3 = TreeNode(1)
      >>> tree3.left = TreeNode(2)
      >>> tree3.right = TreeNode(3)
      >>> tree4 = TreeNode(1)
      >>> tree4.left = TreeNode(3)
      >>> tree4.right = TreeNode(2)
      >>> are_similar(tree3, tree4)
      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)