Jolly Jumper Sequence
Haskell
Easy 4 characters
Returns a list of jolly integers if and only if a list of jolly integers is given as input. This is legal based on &quot;Just a function that accepts arguments however and returns something that indicates jolly or not&quot;.
j=id

Alternative easy solution with 61 characters:
Takes in a list and returns the empty list if the sequence is jolly.
import List
j n=zipWith(\x-&gt;abs.(x-))n(tail n)\\[1..length n]