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 Sign of a Permutation (Posted on 2006-08-05)
A permutation p1,p2,...,pn of 1,2,...,n is even (resp. odd) if it can be returned to the original order 1,2,...,n using an even (resp. odd) number of interchanges of pairs of elements. It is known that every permutation is either even or odd (and therefore not both).

The sign, or signum (for those who want to be fancy), of a permutation is +1 for an even permutation and -1 for an odd permutation.

You are given the integer n and the array p initialized to the integer values p[1]=p1, p[2]=p2, ..., p[n]=pn which are guaranteed to be a permutation of 1,2,...,n. What algorithm would you advocate for determining the sign of the permutation? You need not preserve the original contents of p.

 See The Solution Submitted by Richard Rating: 3.2000 (5 votes)

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Count how many times P[I]>P[J] with I<J, and see if the count is odd or even.
 Posted by Federico Kereki on 2006-08-05 19:09:26

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