(MEDIUM) Given an input String, rearrange the letters so that pairs of letters are moved from the back to the front. The first two letters are swapped with the last two, though both pairs stay in their original order. Then the next two letters from each direction (front and back) are swapped, and so on. A String with original index positions of 01234567 becomes 67452301. This encryption scheme is also its own decryption scheme, since running it with previous output will restore the original String. Precondition: input Strings will have a length evenly divisible by 2.

HINT

For many String construction problems, the strategy is to build an output String one piece at a time by concatenating characters or other Strings into it. You can create an empty String like this:

String a = "";

Since the output of this problem is built two characters at a time, maybe you could design a loop and a substring that think about two characters at once?