Inversion Counting

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Count the number of inversions in an array. An inversion is two items that are out of order. For instance, in the array: [1][3][2] the numbers 3 and 2 represent an inversion.

This algorithm is essentially the same as merge sort with the exception that it keeps track of inversions while sorting.


The typical use of such algorithms is comparing preferences between users. The number of inversions between user A’s ranking of products and users B’s ranking of products represents how similar their affinities are.

Asymptotic Complexity

$O(n \log n)$

Pseudo Code

A = input array
inversions = 0

if length of A is 1
    return 0

    A1 = first half of A
    A2 = second half of A

    inversions += recursively count\sort A1
    inversions += recursively count\sort A2

    i = 1
    j = 1

    for k = 1 to n
        if A1[i] < A2[j]
            A[k] = A1[i]
            A[k] = A2[j]
            inversions += # of items remaining in A1

    return inversions

Source Code

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