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Editorial Team
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Asked: 2026-06-09T15:21:29+00:00

I am trying to study various sorting and searching algorithms for an upcoming test

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I am trying to study various sorting and searching algorithms for an upcoming test this Tuesday. All went well until I got to the Quick Sort Algorithm. I did not have a book or any other resource, so I went online and started reading the SparkNote. I thought I understood the text, and I even read the Quick Sort Algorithm part of a PowerPoint I found online.

However, SparkNote provided an example on the page of the step-by-step process of the algorithm, but it did not show the steps to initially arrange the list. The list given was [5 9 3 8 6 4 2 1 7 0]. According to SparkNotes, the arranged list, with the values less than the pivot (which is 5) on the left and the values greater than the pivot on the right, is [0 3 4 2 1 5 8 6 7 9]. However, when I try to go through the steps myself, I keep getting [ 4 0 3 1 2 5 6 8 7 9 ].

The procedure I took was:

5 9 3 8 6 4 2 1 7 0 // The initial list. Pivot = 5
5 0 3 8 6 4 2 1 7 9 // Switched 0 and 9.
5 0 3 1 6 4 2 1 7 9 // Switched 8 and 1
5 0 3 1 2 4 6 8 7 9 // Switched 6 and 2
4 0 3 1 2 5 6 8 7 9 // Switched 4 and 5 because the lines that point to the 
                    // greater and smaller numbers crossed.

Where is my mistake? Also, I see that the numbers less than 5 are on the left and the numbers greater than 5 are on the right, so, does my mistake really affect the sorting?

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1 Answer

  1. Editorial Team

    The algorithm described in SparkNotes initially places the pivot element to the rightmost position in the array. The algorithm you used placed/kept the pivot to the leftmost position. No wonder the arrangements after partitioning are different.

    That means that they started with 

    5 9 3 8 6 4 2 1 7 0

    chose 5 as pivot and placed it in the rightmost position (exchanged 5 and 0)

    0 9 3 8 6 4 2 1 7 5

    and only after that they performed partitioning for the remaining elements.

    You kept your 5 in the leftmost position (apparently you simply forgot to do step 2 from SparkNotes). In the end, both variants work, i.e. there’s no “mistake”. In your case the arrangement is perfectly valid with the array correctly partitioned.

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