input: Write a function to determine if two binary trees are similar. Two trees are considered similar if they have the same structure and corresponding values. If both roots are None, they are considered similar. The function should take two TreeNode objects as input and return True if the trees are similar, otherwise return False.
  
  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), None) == False
  assert are_similar(None, TreeNode(1)) == False
  assert are_similar(TreeNode(1), TreeNode(1)) == True
  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
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)