input: Write a function to compare two binary trees and check if they are similar. If both trees are the same in structure and node values, return True, otherwise return False. Define the TreeNode class as given and use the function to check similarity.
  
  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) != (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
  root1 = TreeNode(1)
  root1.left = TreeNode(2)
  root1.right = TreeNode(3)
  
  root2 = TreeNode(1)
  root2.left = TreeNode(2)
  root2.right = TreeNode(3)
  
  root3 = TreeNode(1)
  root3.left = TreeNode(3)
  root3.right = TreeNode(2)
  
  root4 = TreeNode(1)
  root4.left = TreeNode(2)
  
  assert are_similar(root1, root2)==True
  assert are_similar(root1, root3)==False
  assert are_similar(root1, root4)==False
  assert are_similar(None, None)==True
output: def are_similar(root1, root2):
      if root1 is None and root2 is None:
          return True
      if (root1 is None) != (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)