input: Write a function to determine if two binary trees are similar. Similar binary trees have the same structure and the same values at each corresponding node position. If the trees are similar, return True, otherwise return False. If both roots are None, they are considered similar.
  
  class TreeNode:
      def __init__(self, x):
          self.val = x
          self.left = None
          self.right = None
  
  def are_similar(root1, root2):
      if root1 is None and root2 is None:
          return True
      if root1 is None or root2 is None:
          return False
      if root1.val != root2.val:
          return False
      return are_similar(root1.left, root2.left) and are_similar(root1.right, root2.right)
  
  assert are_similar(None, None) == True
  assert are_similar(TreeNode(1), TreeNode(1)) == True
  assert are_similar(TreeNode(1), TreeNode(2)) == False
  root1 = TreeNode(1)
  root1.left = TreeNode(2)
  root1.right = TreeNode(3)
  root2 = TreeNode(1)
  root2.left = TreeNode(2)
  root2.right = TreeNode(3)
  assert are_similar(root1, root2) == True
  root2.left.val = 4
  assert are_similar(root1, root2) == False
output: def are_similar(root1, root2):
      if root1 is None and root2 is None:
          return True
      if root1 is None or root2 is None:
          return False
      if root1.val != root2.val:
          return False
      return are_similar(root1.left, root2.left) and are_similar(root1.right, root2.right)