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.
A fix using Tail Recursion
In Scala, tail recursion enables you to rewrite a mutable structure such as a while-loop, into an immutable algorithm.
So, so long as you can rewrite a recursive function into an iterative function, you're good to go!
However, do not fret, there is a way to rewrite even recursive functions into stack-safe ones, called 'Defunctionalisation', which we must cover in future.
A fix using foldLeft and foldRight
Tail recursion can be expressed in FoldLeft-FoldRight, and vice-versa; which is better depends on how it looks in the end. It's also related to State machine
A fix using Range
Here, we benefit from Scala's utilities that allow specialisation
A fix using a direct formula
Ideal if you can avoid an iteration and come up with a formula that runs in O(1)