input: def are_similar(root1, root2):
      """
      Check if two binary trees are similar by comparing their structure and values.
      The trees are similar if they have the same structure and the same values at corresponding nodes.
      
      >>> t1 = TreeNode(1)
      >>> t1.left = TreeNode(2)
      >>> t1.right = TreeNode(3)
      >>> t2 = TreeNode(1)
      >>> t2.left = TreeNode(2)
      >>> t2.right = TreeNode(3)
      >>> are_similar(t1, t2)
      True
      
      >>> t3 = TreeNode(1)
      >>> t3.left = TreeNode(2)
      >>> t4 = TreeNode(1)
      >>> t4.right = TreeNode(2)
      >>> are_similar(t3, t4)
      False
      """
output:     if not root1 and not root2:
          return True
      if not root1 or not root2:
          return False
      if root1.val != root2.val:
          return False
      return are_similar(root1.left, root2.left) and are_similar(root1.right, root2.right)