// AP Computer Science, AB Exam, 1996, Question 3, part B in C++

bool IsBST(TreeNode * t)
// postcondition: returns true if t represents a binary search
//                tree containing no duplicate values;
//                otherwise, returns false.
{
    if (t == NULL) return true;          // empty tree is a search tree

    return ValsLess(t->left,t->info)     &&
	   ValsGreater(t->right,t->info) &&
	   IsBST(t->left)                &&
	   IsBST(t->right);
}