input: Write a function to check if two binary trees are similar. The function should return True if the trees are similar, otherwise return False. If both trees are empty, return True.
  
  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)
  
  # Test cases
  assert are_similar(None, None) == True
  assert are_similar(TreeNode(1), TreeNode(1)) == True
  assert are_similar(TreeNode(1), TreeNode(2)) == False
  assert are_similar(TreeNode(1, TreeNode(2), TreeNode(3)), TreeNode(1, TreeNode(2), TreeNode(3))) == True
  assert are_similar(TreeNode(1, TreeNode(2), TreeNode(3)), TreeNode(1, TreeNode(3), TreeNode(2))) == False
  assert are_similar(TreeNode(1, TreeNode(2), None), TreeNode(1, TreeNode(2), TreeNode(3))) == 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)