题解 | #判断是不是二叉搜索树#

from re import T
import sys
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
#
# @param root TreeNode类
# @return bool布尔型
#
class Solution:
    def isValidBST(self, root: TreeNode) -> bool:
        # write code here
		'''
		中序遍历收集序列的值，如果值递增为二叉排序树
		'''
        pre=-sys.maxsize-1
        res=[]
        def middletravel(root,pre):
            #re=pre
            if not root:
                return True

            middletravel(root.left,pre)
            #if root.val <pre:
            #    return False
            res.append(root.val)
        
            pre=root.val
            middletravel(root.right,pre)
        middletravel(root,pre)
        for i in range(len(res)-1):
            if res[i+1]<res[i]:
                return False
        return True