本文共 375 字，大约阅读时间需要 1 分钟。

class Solution:    def isValidBST(self, root: TreeNode) -> bool:        stack = []        pre = float('-inf')        p = root        while stack or p:            while p:                stack.append(p)                p = p.left            p = stack.pop()            if p.val <= pre:                return False            pre = p.val            p = p.right        return True