Diameter of a binary tree

class Solution:
    def diameterOfBinaryTree(self, root: Optional[TreeNode]) -> int:

        global diameter
        diameter = -float("inf")

        def helper(root):

            global diameter
            if not root:
                return 0

            left_path = helper(root.left)
            right_path = helper(root.right)

            diameter = max(diameter, left_path + right_path)

            return 1 + max(left_path, right_path)

        helper(root)
        return diameter