leetcode之Validate Binary Search Tree

这道题判断一个二叉树是不是BST。用中序遍历成为一个数组，看是不是每个值都比后面小就好了。代码如下：

# Definition for a binary tree node.# class TreeNode(object):#     def __init__(self, x):#         self.val = x#         self.left = None#         self.right = Noneclass Solution(object):    def isValidBST(self, root):        """        :type root: TreeNode        :rtype: bool        """        a = []        def search(root):            if root == None:                return True            if root.left != None:                search(root.left)            a.append(root.val)            if root.right != None:                search(root.right)        search(root)        print a        if len(a) == 1:            return True        for i in range(len(a) -1):            if a[i] >= a[i + 1]:                return False        else:            return True