input: Write a function to check if two binary trees are similar. Two binary trees are considered similar if they have the same structure and the same values at each corresponding node. If both roots are None, return True. If only one root is None, return False. Otherwise, compare the values of the roots and recursively check the left and right subtrees.
  
  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 and root2) is None or (root1 or root2) is None:
          return False
      if root1.val != root2.val:
          return False
      else:
          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)