Valid Binary Search Tree

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

Assume a BST is defined as follows:

1. The left subtree of a node contains only nodes with keys less than the node’s key.
2. The right subtree of a node contains only nodes with keys greater than the node’s key.
3. Both the left and right subtrees must also be binary search trees.

Example :

Input : 
   1
  /  \
 2    3

Output : 0 or False


Input : 
  2
 / \
1   3

Output : 1 or True 

Return 0 / 1 ( 0 for false, 1 for true ) for this problem

SOLUTION APPROACH :

Complete solution in the hints.