Validate Binary Search Tree

Problem

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

• 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.

Example

An example:

      2
     / \
    1   4
       / \
      3   5

The above binary tree is serialized as {2,1,4,#,#,3,5} (in level order).