判断是否是二叉搜索树

import sys

class Node(object):

    def __init__(self,val=None):

        self.val = val

        self.left = None

        self.right = None

 

#非递归版本

#中序遍历把打印的时机换成比较

 

def isBST(head):

    if not head:

        return True

    stack = []

    pre = -sys.maxsize

    while stack or head != None:

        if head != None:

            stack.append(head)

            head = head.left

        else:

            head = stack.pop()

            print(head.val, end=' ')

            if head.val>pre:

                pre = head.val

                head = head.right

            else:

                return False

    return True