Create Validate Binary Search Tree 

Validate Binary Search Tree Total Accepted: 21394 Total Submissions: 82668 My Submissions
Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

BST 要求左边都要小于根，右边的都要大于根。


The left subtree of a node contains only nodes with keys less than the node's key.
The right subtree of a node contains only nodes with keys greater than the node's key.
Both the left and right subtrees must also be binary search trees.
confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.