Why Greed Works for Shortest Common Superstring Problem

  • Authors:
  • Bin Ma

  • Affiliations:
  • Department of Computer Science, University of Western Ontario, London, Canada N6A 5B7

  • Venue:
  • CPM '08 Proceedings of the 19th annual symposium on Combinatorial Pattern Matching
  • Year:
  • 2008

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Abstract

The shortest common superstring problem (SCS) has been widely studied for its applications in string compression and DNA sequence assembly. Although it is known to be Max-SNP hard, the simple greedy algorithm works extremely well in practice. Previous researchers have proved that the greedy algorithm is asymptotically optimal on random instances. Unfortunately, the practical instances in DNA sequence assembly are very different from random instances.In this paper we explain the good performance of greedy algorithm by using the smoothed analysis. We show that, for anygiven instance Iof SCS, the average approximation ratio of the greedy algorithm on a small random perturbation of Iis 1 + o(1). The perturbation defined in the paper is small and naturally represents the mutations of the DNA sequence during evolution.Due to the existence of the uncertain nucleotides in the output of a DNA sequencing machine, we also proposed the shortest common superstring with wildcards problem (SCSW). We prove that in worst case SCSW cannot be approximated within ratio n1/7 茂戮驴 茂戮驴, while the greedy algorithm still has 1 + o(1) smoothed approximation ratio.