For a comparison based sorting algorithm, it is desirable to:

  1. Be stable
  2. Run in $O(n\ \log n)$
  3. Operate in $O(1)$ space

I can find algorithms that satisfy any two of these but I can't find any that satisfy all three. Does it exist? Is it proven not to exist?


1 Answer 1


Mergesort satisfies all three requirements (when merging is performed in place). See Pardo, L.T., "Stable sorting and merging with optimal space and time bounds", SIAM J. Comput. 6 (1977), 351-372.

  • $\begingroup$ I think that the algorithm uses $O(1)$ pointers, but has a recursive call stack that is logarithmic in the size of the list. Am I mistaken? $\endgroup$
    – aelguindy
    Apr 11, 2012 at 11:54
  • 6
    $\begingroup$ Mergesort can be implemented without recursion, essentially by doing a bottom-up level-order traversal of the recursion tree. $\endgroup$
    – Jeffε
    Apr 11, 2012 at 12:15
  • 1
    $\begingroup$ there is a nice paper by Salowe and Steiger that describes some background and gives simplified algorithms: dx.doi.org/10.1016/0196-6774(87)90050-2 $\endgroup$ Apr 13, 2012 at 15:34
  • $\begingroup$ Here is an implementation of such a sort, along with a lot of documentation of how it's implemented: github.com/BonzaiThePenguin/WikiSort/tree/master $\endgroup$ Jun 27, 2014 at 13:53

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