A binary tree is binary search tree (BST) if :

• If the left subtree nodes value is less than the given node.
• If the right subtree nodes value is greater than the give node
• Both the left and right subtrees are BST.

public class BinaryTreeIsBST {

	public boolean IsBST(TreeNode node, int max, int min){

		if(node == null)
			return true;
		if(min < node.value && node.value > max){
			return IsBST(node.left,min,node.value) &&
                         IsBST(node.right,node.value,max);
		}
		return false;
	}
}