Where Quicksort uses the recursion stack to maintain the `intervals` implicitly, Lampsort instead uses some set data structure to maintain this set of subproblems explicitly.

It reminds me of the way you turn recursive depth-first search (recurse on children that have not been visited) into non-recursive depth-first search (push the children that have not been visited onto the DFS stack). Calling it "non-recursive" is a bit misleading, since you're trading recursion stack space for an actual data stack to maintain the recursion you are supposed to avoid -- but it is a useful program transformation to know.