check whether a tree is BST or not(3)

#include<iostream>
using namespace std;

class Node{
  public:
      int data;
      Node *left,*right;
      Node(int key){
        data=key;
        left=right=NULL;
        }
  };

int Check(Node * root,int minimum,int maximum){
  if(root==NULL)
     return 1;
  if(minimum>root->data&& maximum<root->data)
    return 0;
 return Check(root->left,minimum,root->data-1) && Check(root->right,root->data+1,maximum);
  }

int CheckBST(Node* root){
  if(root==NULL)
     return 1;
  return Check(root,INT_MIN,INT_MAX);
   }

void Insert(Node **root_ref,int key){
  if(*root_ref==NULL){
    *root_ref=new Node(key);
    return;
    }
  if((*root_ref)->data<key)
    Insert(&((*root_ref)->right),key);
  else
     if((*root_ref)->data>key)
       Insert(&((*root_ref)->left),key);
  }

int main(){
 Node *root=NULL;
 Insert(&root,5);
 Insert(&root,7);
 Insert(&root,2);
 Insert(&root,8);
 if(CheckBST(root))
    cout<<" yes it is a BST ";
 else
    cout<<" no ";
 }