Question

I was looking at Wikipedia’s pseudo-code (and other webpages like sortvis.org and sorting-algorithm.com) on merge sort and saw the preparation of a merge uses recursion.

I was curious to see if there is a non-recursive way to do it.

Perhaps something like a for each i element in list, i=[i-th element].

I am under the impression that recursion is keep-it-to-a-minimum-because-it’s-undesirable, and so therefore I thought of this question.

The following is a pseudo-code sample of the recursive part of the merge-sort from Wikipedia:

function merge_sort(list m)
    // if list size is 1, consider it sorted and return it
    if length(m) <= 1
        return m
    // else list size is > 1, so split the list into two sublists
    var list left, right
    var integer middle = length(m) / 2
    for each x in m up to middle
         add x to left
    for each x in m after or equal middle
         add x to right
    // recursively call merge_sort() to further split each sublist
    // until sublist size is 1
    left = merge_sort(left)
    right = merge_sort(right)

Answer 1

Bottom-up merge sort is a non-recursive variant of merge sort.

See also this wikipedia page for a more detailed pseudocode implementation.