98. 验证二叉搜索树 - 力扣（LeetCode）
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:def isValidBST(self, root: Optional[TreeNode]) -> bool:def helper(node, min_val=float('-inf'), max_val=float('inf')):if not node:return Trueif not (min_val < node.val < max_val):return Falsereturn helper(node.left, min_val, node.val) and helper(node.right, node.val, max_val)return helper(root)
