input: def are_similar(root1, root2):
      """
      Compares two binary trees to check if they are similar. Two binary trees are similar if they have the same structure and the same values at corresponding nodes.
      Input: root1, root2 - the root nodes of the two binary trees to be compared
      Output: a boolean indicating whether the two trees are similar or not
      
      >>> 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)
      >>> are_similar(tree1, tree3)
      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)