Homework: Double Up Null

Created By: Chris Taylor

/ Version: 2023.6.0

Write a method, doubleUp, that accepts a List<String>? and returns a List<String>? where each String in the original list is concatenated with itself. If a String in the list is null, simply pass it through, as shown below. If the passed List is null, return null. Your solution must not use loops and must use at least one lambda expression.

• ["a", "b"] -> ["aa", "bb"]
• ["", null] -> ["", null]
• null -> null