Question

If I am given three functions:
1) empty – returns whether a passed list is empty
2) split – splits one list into two equal sized lists
3) concat – concatenates two lists into one

Using only these three functions, how can I reverse a list?

Answer 1

Split list in two, recursively reverse sublists, and merge them backwards.

def reverse(x) {
    y, z = split(x)
    if (empty(y) || empty(z)) 
        return x // list has only one element
    else
        return concat(reverse(z), reverse(y))

}