If a linked list is doubly linked, then deleting a block pointed to by bp takes constant time:

        after = NEXT_FREE(bp);
        before = PREV_FREE(bp);
        PUT_NEXT(before, after);
        PUT_PREV(after, before);
     

We would need to write the four macros:

• NEXT_FREE
• PREV_FREE
• PUT_NEXT
• PUT_PREV