Verify a binary tree is a binary search tree

Algorithm: In-order traversal the binary tree and examine that the visited node’s data are in sorted order, then the binary tree is BST.

Here is source code:

#include <stdlib.h>
#include <stdio.h>
#include <limits.h>

struct node{
   int data;
   node* leftchild;
   node* rightchild;
   node(int d):data(d), leftchild(NULL),rightchild(NULL){}
};

bool isBST(node* ptr)
{
   static node* prv_node = new node(INT_MIN);
   
   if(ptr)
   {
      if(!isBST(ptr->leftchild)) return false;
      
      if(prv_node->data >= ptr->data) return false;
      
      prv_node = ptr;
      
      if(!isBST(ptr->rightchild)) return false;
   }
   
   return true;
} 

int main()
{
   /*input: a binary tree
               5
              / \
             3   6
            / \ 
           2   4
   */
   node* root = new node(1);
   root->leftchild = new node(3);
   root->rightchild = new node(6);
   root->leftchild->leftchild = new node(2);
   root->leftchild->rightchild = new node(4);

   if(isBST(root)) printf("is BST\n");
   else printf("not BST\n");
   
   /*output:
     is BST
   */
}