Introduction to Algorithms, Second Edition

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The University of Texas at Austin Lecture 7 Department of Computer Sciences Professor Vijaya Ramachandran Randomized Select CS357: ALGORITHMS, Spring 2006 Randomized Selection The selection problem (Chapter 9) is the following. Input. An array A [1 ..n ] of n distinct elements from a totally ordered set, and an index i , where 1 i n . Output. The i th smallest element in array A . If i = 1 the output is the minimum element in the array, and if i = n the output is the maximum element in the array. The median element is the output if n is odd and i = ( n + 1) / 2. If n is even there are two ‘middle elements’ corresponding to i = n/ 2 and i = ( n/ 2) + 1; for convenience we will assume that the median is i = d ( n + 1) / 2 e , i.e., the upper middle element, when n is even. The selection problem can be solved in O ( n log n ) time using a good sorting algorithm. But it can be solved in O ( n ) expected time using the follow- ing randomized algorithm. (It can also be solved deterministically in O ( n ) worst-case time, but we will not go into that here.)
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  • Spring '06
  • Ramachandran
  • Algorithms, xk, ith smallest element, Professor Vijaya Ramachandran

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