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Algorithm Problem - Counting Number of Inversions

We are given a sequence ofndistinct numbers a1,...,an. We define an inversion to be a pair i<j such that ai>aj. Give an O(nlogn) algorithm to count the number of inversions. (Hint: assume that your algorithmreturns a sorted array of input sequences, as well as the number of inversions. Divide the input sequencesinto two halves, then count inversions and sort each half by doing recursive calls, and find a way to mergethese two sorted halves and to count the number of inversion between them.)

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