Solve: Double Upper

Created By: Chris Taylor

/ Version: 2023.7.0

Write a recursive method, doubleUpper, that accepts a list of Strings and returns a list of Strings where each String in the original list is concatenated with itself. You may assume that the list passed is not null and does not contain any null elements.

• ["a", "b"] -> ["aa", "bb"]
• ["", "last"] -> ["", "lastlast"]