A three-string approach to the closest string problem

  • Authors:
  • Zhi-Zhong Chen;Bin Ma;Lusheng Wang

  • Affiliations:
  • Department of Mathematical Sciences, Tokyo Denki University, Hatoyama, Saitama, Japan;School of Computer Science, University of Waterloo, Waterloo, ON, Canada;Department of Computer Science, City University of Hong Kong, Hong Kong SAR

  • Venue:
  • COCOON'10 Proceedings of the 16th annual international conference on Computing and combinatorics
  • Year:
  • 2010

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Abstract

Given a set of n strings of length L and a radius d, the closest string problem asks for a new string tsol that is within a Hamming distance of d to each of the given strings. It is known that the problem is NP-hard and its optimization version admits a polynomial time approximation scheme (PTAS). Parameterized algorithms have been then developed to solve the problem when d is small. In this paper, with a new approach (called the 3-string approach), we first design a parameterized algorithm for binary strings that runs in O(nL + nd36.731d) time, while the previous best runs in O(nL + nd8d) time. We then extend the algorithm to arbitrary alphabet sizes, obtaining an algorithm that runs in O(nL + nd1.612d(α2 + 1 - 2α-1 + α-2)3d time, where α = 3√√|Σ| - 1 + 1. This new time bound is better than the previous best for small alphabets, including the very important case where |Σ| = 4 (i.e., the case of DNA strings).