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 = None

class 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