determine if a given binary tree is a valid bst

// Java implementation to check if given Binary tree 
// is a BST or not 
  
/* Class containing left and right child of current 
 node and key value*/
class Node 
{ 
    int data; 
    Node left, right; 
  
    public Node(int item) 
    { 
        data = item; 
        left = right = null; 
    } 
} 
  
public class BinaryTree 
{ 
    // Root of the Binary Tree 
    Node root; 
  
    // To keep tract of previous node in Inorder Traversal 
    Node prev; 
  
    boolean isBST()  { 
        prev = null; 
        return isBST(root); 
    } 
  
    /* Returns true if given search tree is binary 
       search tree (efficient version) */
    boolean isBST(Node node) 
    { 
        // traverse the tree in inorder fashion and 
        // keep a track of previous node 
        if (node != null) 
        { 
            if (!isBST(node.left)) 
                return false; 
  
            // allows only distinct values node 
            if (prev != null && node.data <= prev.data ) 
                return false; 
            prev = node; 
            return isBST(node.right); 
        } 
        return true; 
    } 
  
    /* Driver program to test above functions */
    public static void main(String args[]) 
    { 
        BinaryTree tree = new BinaryTree(); 
        tree.root = new Node(4); 
        tree.root.left = new Node(2); 
        tree.root.right = new Node(5); 
        tree.root.left.left = new Node(1); 
        tree.root.left.right = new Node(3); 
  
        if (tree.isBST()) 
            System.out.println("IS BST"); 
        else
            System.out.println("Not a BST"); 
    } 
}