An Optimal Offline Algorithm for List Update

Abstract

For the static list update problem, given an ordered list ${\rho }_0$ (an ordering of the list $L$ = \{ $a_a,a_2,...,a_l$ \}), and a sequence $\sigma =({\sigma }_1,{\sigma }_2,...,{\sigma }_m)$ of requests for items in $L$, we characterize the list reorganizations in an optimal offline solution in terms of an initial permutation of the list followed by a sequence of $m$ {\em element transfers}, where an element transfer is a type of list reorganization where only the requested item can be moved. Then we make use of this characterization to design an $O(l^2(l-1)!m)$ time optimal offline algorithm.