Validate Binary Search Tree

	/**
	 * 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.
	 * 
	 * */