input: Write a function to determine if two binary trees are similar. Two binary trees are considered similar if they have the same structure and the same values at corresponding nodes. The function should return True if the trees are similar and False otherwise. 
  
  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
  
  root1 = TreeNode(1)
  root1.left = TreeNode(2)
  root1.right = TreeNode(3)
  
  root2 = TreeNode(1)
  root2.left = TreeNode(2)
  root2.right = TreeNode(3)
  
  assert are_similar(root1, root2)==True
  
  root3 = TreeNode(1)
  root3.left = TreeNode(3)
  root3.right = TreeNode(2)
  
  assert are_similar(root1, root3)==False
  
  root4 = TreeNode(1)
  root4.left = TreeNode(2)
  
  root5 = TreeNode(1)
  root5.right = TreeNode(2)
  
  assert are_similar(root4, root5)==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)