98. 验证二叉搜索树

class Solution:
    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        if not root:
            return True

        pre = None
        stack = list()
        while root or stack:
            if root:
                stack.append(root)
                root = root.left
            else:
                root = stack.pop()
                if pre and root.val <= pre.val:
                        return False
                
                pre = root
                root = root.right

        return True