Check if binary tree is BST or not.

BinaryBST.cpp

#include<iostream>
#include<bits/stdc++.h>
using namespace std;
class node{
    public: int data;
    node* left;
    node* right;
node(int data)
{
    this->data = data;
    this->left = NULL;
    this->right = NULL;
}
};
int isBST(node* root){
    static node *prev = NULL;

    // traverse the tree in inorder fashion and keep track of prev node
    if (root)
   {
        if (!isBST(root->left))
           return 0;

        // Allows only distinct valued nodes 
        if (prev != NULL && root->data <= prev->data)
               return 0;

          prev = root;

        return isBST(root->right);
  }

   return true;
    
    
}

int main()
{
    node* root = new node(4);
    root->left = new node(2);
    root->right = new node(5);
    root->left->right = new node(1);
    root->left->right=new node(3);
    
    if(isBST(root))
        cout<<"is BST";
    else cout<<"is not BST";
    
    return 0;
}