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Editorial Team
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Asked: 2026-05-18T07:28:35+00:00

In Introduction to Algorithms the merge sort algorithm is implemented with a helper function

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In “Introduction to Algorithms” the merge sort algorithm is implemented with a helper function called MERGE(A, p, q, r) – that is merging two previously sorted sequences .

This function introduces two additional arrays L and R . 

MERGE(A, p, q, r)
1 n1 ← q − p + 1
2 n2 ←r − q
3 create arrays L[1 . . n1 + 1] and R[1 . . n2 + 1]
.....

By "create arrays L[1 . . n1 + 1] and R[1 . . n2 + 1]" I understand to allocate additional memory for both of them .

Is it possible to re-write this function, so that I won’t need the additional memory, and to operate directly to A ?

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1 Answer

  1. Editorial Team

    Sure. It is called in-place merge sort.

    Wikipedia say it is complicated — but it is not always true. Some are not as complicated as others, if you don’t care about the run-time.

    There are a few variance, some are stable, some are non-stable. See the “implementation” section under NIST DIAGS for some example.

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