gistfile1.txt

public static boolean isBST(Node head){
  if(head == null){
    return false;
  }else{
    return isBST(head, Integer.MIN_INTEGER, Integer.MAX_INTEGER);
  }
}

private static boolean isBST(Node head, int min, int max){
  if(head == null){
    return true;
  }
  
  if(head.val <= max && head.val > min){
    return isBST(head.left, min, head.val) && isBST(head.right, head.val, max);
  }else{
    return false;
  }
}