def check_binary_search_tree_(root):
    # create helper function
    def check(root, min_value, max_value):
        # base case
        if root is None:
            return True
        
        # general case
        if root.data < min_value or root.data > max_value:
            return False
        
        return check(root.left, min_value, root.data - 1) and check(root.right, root.data + 1, max_value)
    
    return check(root, 0, 10000)