is there any trick inside?
it seems easy...but i always get wrong answer...@@

What data structure do you use?
I wrote
(heap of free) + (double linked list sorted by access time)
very quickly, but probably wrong.
Is there a simplier way?

I used 2 heaps (of free and of access times) dynamicaly
changing the heap of access times when needed with
reference table array.(it worked)