Average case selection

  • Authors:
  • Walter Cunto;J. Ian Munro

  • Affiliations:
  • Univ. Simón Bolívar, Caracas, Venezuela;Univ. of Waterloo, Waterloo, Ont., Canada

  • Venue:
  • Journal of the ACM (JACM)
  • Year:
  • 1989

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Abstract

It is shown that n + k - O(1) comparisons are necessary, on average, to find the kth smallest of n numbers (k ⪇ n/2). This lower bound matches the behavior of the technique of Floyd and Rivest to within a lower-order term. 7n/4 ± o(n) comparisons, on average, are shown to be necessary and sufficient to find the maximum and median of a set. An upper bound of 9n/4 ± o(n) and a lower bound of 2n - o(n) are shown for the max-min-median problem.