OJ Board

thank you,Divid.
what a bright idea! and i am really dull. I once thought the comparison should be based on sets(the string I've already got,i mean),but I could'nt go further...
thanks a lot.

I,ve searched the Board,some body said using dp,and some said "clever sorting"(I guess that's using greedy method), but I need more clue....

I think Dp or greedy are both OK,but I don't know how to.
thank you.