Given a Binary Tree check whether it is Binary Search Tree or not

Write an isBST() function that returns true if a tree is a binary search tree
and false otherwise.

/*
 * Is BST helper function
 */
int isBSTUtil(nodeptr node, int min, int max)
{
    if(node != NULL)
        return (node->data >= min && node->data <=max) &&
            isBSTUtil(node->left, min, node->data) &&
            isBSTUtil(node->right, node->data, max);
    return 1;
           
}
/*
 * determine given binary tree is BST
 */
int isBST(nodeptr node)
{
    int min,max;
    if (node != NULL) {
        min = minValue(node);
        max = maxValue(node);
        return isBSTUtil(node, min, max);
    }
    return 1;
}