Two strings of equal length can be interlaced like the teeth of a zipper to form a new word. For example, interlacing cota and rain yields croatian. The following class constructs a ZipperWord from two strings.
Your task is to complete the toString method using recursion. Hint: Concatenate the first letter from each string, follwed by a simpler ZipperWord that is made from the tails of the strings (first.substring(1) and second.substring(1)).