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 concept of inversion dates back to 1750 when G. Cramer used them as a device for solving linear equations1.


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.

  • Collaborative Filtering

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

Asymptotic Complexity

$O(n \log n)$

Source Code

Full Repo

Relevant Files:

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    a. What is your name?
    b. What is your quest?
    c. What… is the air-speed velocity of an unladen swallow?

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    1. It is Arthur, King of the Britons
    2. To seek the Holy Grail
    3. What do you mean? An African or European swallow?
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    Answers (click to expand)
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  1. The Art of Computer Programming Volume 3 Sorting and Searching p. 11