Question

I need a way to represent information about Weeks and Days as such:

W1: D1,D2
W2: D1,D5
W3: D3,D2

so that, given my current week and day, I should be able to get the previous day and next day

For ex:

if(currentDay = W2D5)
{
    prevDay = W2D1;
    nextDay = W3D3;
}

Here are two options I came up with:

1. Use a treemap

   TreeMap< Integer,TreeMap< Integer>>

In the outer tree map, store the week number as the key and in the inner tree map, store the

1. Use an array
   [w1d1,w2d2,w2d1…]

What would be a good data structure to use for such a problem?

Answer 1

Why not just a well-ordered collection on a WeekDay type? (The WeekDay type includes a Week and a Day and provides well-defined ordering.)

If this were the case, TreeSet<WeekDay> could be used with higher/lower (or even headSet/tailSet).

TreeSet<WeekDay> days = ...;
WeekDay nextDay = days.higher(currentDay);

Unfortunately TreeSet<WeekDay> (or PriorityQueue) and other standard data-types won’t work because they lack a method of iterating forward/back from a given element. This isn’t strictly true, at least TreeMap can be used, just with a O(1) manual look-up using the iterator: however, the required operations to take advantage of the O(lg n) access just aren’t “standard” in a TreeSet.

This article on Sorted Lists in Java covers a bunch of interesting things, as well as providing an implementation which could be adapted.

Of course, it is also possible to just use an ArrayList and keep it sorted on update. In that case a Collections.binarySearch can be used and the index for prev/next are easily provided 🙂

Happy coding.