boolean checkBST(Node root) {
        return recursiveCheck(root, -1, 10001);       
    }

    boolean recursiveCheck(Node root, int min, int max){
        
        if(root == null)
            return true;
    
        if(root.data <= min || root.data >= max)
            return false;

        return(recursiveCheck(root.left, min, root.data) && recursiveCheck(root.right, root.data, max));
                

    }