Creative Commons Attribution 3.0 Unported license
We study sketching and streaming algorithms for the Longest Common Subsequence problem (LCS) on strings of small alphabet size |Sigma|. For the problem of deciding whether the LCS of strings x,y has length at least L, we obtain a sketch size and streaming space usage of O(L^{|Sigma| - 1} log L). We also prove matching unconditional lower bounds.
As an application, we study a variant of LCS where each alphabet symbol is equipped with a weight that is given as input, and the task is to compute a common subsequence of maximum total weight. Using our sketching algorithm, we obtain an O(min{nm, n + m^{|Sigma|}})-time algorithm for this problem, on strings x,y of length n,m, with n >= m. We prove optimality of this running time up to lower order factors, assuming the Strong Exponential Time Hypothesis.
@InProceedings{bringmann_et_al:LIPIcs.FSTTCS.2018.40,
author = {Bringmann, Karl and Chaudhury, Bhaskar Ray},
title = {{Sketching, Streaming, and Fine-Grained Complexity of (Weighted) LCS}},
booktitle = {38th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2018)},
pages = {40:1--40:16},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-093-4},
ISSN = {1868-8969},
year = {2018},
volume = {122},
editor = {Ganguly, Sumit and Pandya, Paritosh},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2018.40},
URN = {urn:nbn:de:0030-drops-99390},
doi = {10.4230/LIPIcs.FSTTCS.2018.40},
annote = {Keywords: algorithms, SETH, communication complexity, run-length encoding}
}