input: def are_similar(root1, root2):
      """Compares two binary trees to check if they are similar.
      Two binary trees are considered similar if they have the same structure
      and their corresponding nodes have the same value.
      
      >>> tree1 = TreeNode(1)
      >>> tree1.left = TreeNode(2)
      >>> tree1.right = TreeNode(3)
      >>> tree2 = TreeNode(1)
      >>> tree2.left = TreeNode(2)
      >>> tree2.right = TreeNode(3)
      >>> are_similar(tree1, tree2)
      True
      
      >>> tree3 = TreeNode(1)
      >>> tree3.left = TreeNode(3)
      >>> tree3.right = TreeNode(2)
      >>> tree4 = TreeNode(1)
      >>> tree4.left = TreeNode(2)
      >>> tree4.right = TreeNode(3)
      >>> are_similar(tree3, tree4)
      False
      """
      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)
output:     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)