Selection and sorting in the "restore" model

Timothy M. Chan, J. Ian Munro, Venkatesh Raman

Research output: Chapter in Book/Report/Conference proceedingConference contribution


We consider the classical selection and sorting problems in a model where the initial permutation of the input has to be restored after completing the computation. While the requirement of the restoration is stringent compared to the classical versions of the problems, this model is more relaxed than a read-only memory where the input elements are not allowed to be moved within the input array. We first show that for a sequence of n integers, selection (finding the median or more generally the k-th smallest element for a given k) can be done in 0(n) time using 0(lg n) words1 of extra space in this model. In contrast, no linear-time selection algorithm is known which uses polylogarithmic space in the read-only memory model. For sorting n integers in this model, we first present an O(n lg n)-time algorithm using O(lg n) words of extra space. When the universe size U is polynomial in n, we give a faster O(n)-time algorithm (analogous to radix sort) which uses O(nε) words of extra space for an arbitrarily small constant ε > 0. More generally, we show how to match the time bound of any word-RAM integer-sorting algorithms using O(nε) words of extra space. In sharp contrast, there is an Ω(n2/S)-time lower bound for integer sorting using O(S) bits of space in the read-only memory model. Extension of our results to arbitrary input types beyond integers is not possible: for "indivisible" input elements, we can prove the same Ω(n2/S) lower bound for sorting in our model. En route, we develop linear-time in-place algorithms to extract leading bits of the input array and to compress and decompress strings with low entropy; these techniques may be of independent interest.

Original languageEnglish (US)
Title of host publicationProceedings of the 25th Annual ACM-SIAM Symposium on Discrete Algorithms, SODA 2014
PublisherAssociation for Computing Machinery
Number of pages10
ISBN (Print)9781611973389
StatePublished - 2014
Externally publishedYes
Event25th Annual ACM-SIAM Symposium on Discrete Algorithms, SODA 2014 - Portland, OR, United States
Duration: Jan 5 2014Jan 7 2014

Publication series

NameProceedings of the Annual ACM-SIAM Symposium on Discrete Algorithms


Other25th Annual ACM-SIAM Symposium on Discrete Algorithms, SODA 2014
Country/TerritoryUnited States
CityPortland, OR

ASJC Scopus subject areas

  • Software
  • General Mathematics


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