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, Ishizaka, Hatoyama, Hiki, Saitama, 359-0394, Japan;School of Computer Science, University of Waterloo, 200 University Ave. W, Waterloo, ON, Canada N2L3G1;Department of Computer Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong

  • Venue:
  • Journal of Computer and System Sciences
  • Year:
  • 2012

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Abstract

Given a set of n strings of length L and a radius d, the closest string problem (CSP for short) asks for a string t"s"o"l 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+nd^3@?6.731^d) time, while the previous best runs in O(nL+nd@?8^d) time. We then extend the algorithm to arbitrary alphabet sizes, obtaining an algorithm that runs in time O(nL+nd@?(1.612(|@S|+@b^2+@b-2))^d), where |@S| is the alphabet size and @b=@a^2+1-2@a^-^1+@a^-^2 with @a=|@S|-1+13. This new time bound is better than the previous best for small alphabets, including the very important case where |@S|=4 (i.e., the case of DNA strings).