Check whether a given tree is a BST

/*
 Returns true if a binary tree is a binary search tree.
*/
int isBST(struct node* node) {
  if (node==NULL) return(true);
  // false if the max of the left is > than us

  // (bug -- an earlier version had min/max backwards here)
  if (node->left!=NULL && maxValue(node->left) > node->data)
    return(false);

  // false if the min of the right is <= than us
  if (node->right!=NULL && minValue(node->right) <= node->data)
    return(false);

  // false if, recursively, the left or right is not a BST
  if (!isBST(node->left) || !isBST(node->right))
    return(false);

  // passing all that, it's a BST
  return(true);
} 

OR

/*
 Returns true if the given tree is a binary search tree
 (efficient version).
*/
int isBST2(struct node* node) {
  return(isBSTUtil(node, INT_MIN, INT_MAX));
}
/*
 Returns true if the given tree is a BST and its
 values are >= min and <= max.
*/
int isBSTUtil(struct node* node, int min, int max) {
  if (node==NULL) return(true);

  // false if this node violates the min/max constraint
  if (node->data<min || node->data>max) return(false);

  // otherwise check the subtrees recursively,
  // tightening the min or max constraint
  return
    isBSTUtil(node->left, min, node->data) &&
    isBSTUtil(node->right, node->data+1, max)
  );
}