Verify Preorder Sequence in Binary Search Tree

Given an array of numbers, verify whether it is the correct preorder traversal sequence of a binary search tree.

You may assume each number in the sequence is unique.

Follow up: Could you do it using only constant space complexity?

URL: https://leetcode.com/problems/verify-preorder-sequence-in-binary-search-tree/

import sys
class Solution(object):
    def verifyPreorder(self, preorder):
        """
        :type preorder: List[int]
        :rtype: bool
        """
        stack = []
        root = -sys.maxsize-1

        for entries in preorder:

            if entries < root:
                return False

            while stack != [] and stack[-1] < entries:
                root = stack.pop()

            stack.append(entries)

        return True