4.5 Implement a function to check if a binary tree is a binary search tree.
LeetCode上的原理，请参见我之前的博客Validate Binary Search Tree 验证二叉搜索树。
[CareerCup] 4.5 Validate Binary Search Tree 验证二叉搜索树
原文：http://www.cnblogs.com/grandyang/p/4700781.html
