Non-recursive merge sort.

For those of you who don't like recursion:

http://www.algorithmist.com/index.php/Merge_sort.c

See also:

http://stackoverflow.com/questions/159590/way-to-go-from-recursion-to-iteration

http://stackoverflow.com/questions/1557894/non-recursive-merge-sort

Info and improvements about actual sort used in Java collections:

https://arxiv.org/pdf/1412.0193.pdf