Make a pure-functional queue using stacks (Lists). It would have the following methods:
Try to make it efficient, where possible.
Test cases in Scala
Algorithm in Scala
21 lines of Scala (version 2.13), showing how concise Scala can be!
The most straightforward implementation of a Queue would contain a single List, however in this case, we would have to choose between making
enqueue fast, or making
dequeue fast, because with a List, to get the tail element is an \(O(n)\) operation.
However, there is a more efficient way, which is to use an 'incoming' and an 'outgoing' List: so long as 'outgoing' is not empty, we can destructure it at O(1) cost, and we can enqueue to 'incoming' at O(1) cost. The only time where we would face a cost would be when 'outgoing' is empty, and 'incoming' is non-empty: then, we would have to convert the 'incoming' to 'outgoing' by reversal, which is an O(n) operation. However, amortized, it would be more efficient than the biased approach. (this is © from www.scala-algorithms.com)
Scala concepts & Hints
The 'Option' type is used to describe a computation that either has a result or does not. In Scala, you can 'chain' Option processing, combine with lists and other data structures. For example, you can also turn a pattern-match into a function that return an Option, and vice-versa!
Pattern matching in Scala lets you quickly identify what you are looking for in a data, and also extract it.
Stack safety is present where a function cannot crash due to overflowing the limit of number of recursive calls.
This function will work for n = 5, but will not work for n = 2000 (crash with java.lang.StackOverflowError) - however there is a way to fix it :-)
In Scala Algorithms, we try to write the algorithms in a stack-safe way, where possible, so that when you use the algorithms, they will not crash on large inputs. However, stack-safe implementations are often more complex, and in some cases, overly complex, for the task at hand.
In Scala, tail recursion enables you to rewrite a mutable structure such as a while-loop, into an immutable algorithm.