Question

I had to write a merge sort function in Java. No problem. Well, a little, but I got through it. Then the follow up question I didn’t get.

Question: Given an array A[][] such that A[i][0] is a float and A[i][1] is a nonnegative int giving the multiplicity of the value A[i][0] (here think of a big vector that’s been collapsed down by combining repeated entries and recording how many got combined), write a version of merge sort that returns B[][] where B[i][0] < B[i+1][0] for all i.

Any ideas? The best I could do was merge sort and then group the equal ones, but apparently you can do it all in one step.

Answer 1

Strage question… and using different types in these arrays is just ugly (personal point of view).

However, the most useful thing to do, is to rewrite your merge function with a Comparator.
This way you can sort using whatever property you want.
You would end up with a signature like void merge(A[] arr, Comparator<? super A> comp).
By the way, the Java implementation of sort is a lot like this.

To solve your question you would call:

A[][] a = ...;
merge(a, new Comparator<A[]>() {
  int compare(A[] a, A[] b) {
    return ((Float)a[0]) - ((Float)b[0]);
  }
});