Sorting Data larger than the RAM size

Question

This is a Google interview question: Given 2 machines, each having 64 GB RAM, containing all integers (8 byte), sort the entire 128 GB data. You may assume a small amount of add

Answer 1

ChingPing proposes a O(n log n) sort for each subset, followed by a linear merge (by swapping the elements). The problem with Quicksort (and most of the n log n sorts, is that they require n memory. I'd recommend instead using a SmoothSort which uses constant memory, still runs in O(n log n).

The worst case scenario is where you have something like:

setA = [maxInt .. 1]
setB = [0..minInt]

where both sets are ordered in reverse, but then the merger is in the reverse order.

The (IMO - more clear) explanation of ChingPing's solution is:

Have a pointers 'pointerA', 'pointerB' initialized at the beginning of each array
While setA's pointer is not at the end
  if (setA[pointerA] < setB[pointerB])
    then { pointerA++; }
    else { swap(setA[pointerA], setB[pointerB]); pointerB++; }

The sets should both now be sorted.