input: Write a function to determine if two binary trees are similar (i.e., have the same structure and corresponding nodes have the same values). If they are similar return True else 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 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)
  
  # 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(2)
  root3.right = TreeNode(4)
  
  root4 = TreeNode(1)
  root4.left = TreeNode(2)
  root4.right = None
  
  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 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)