Given 2 strings, a and b, return a string of the form: shorter+reverse(longer)+shorter.
In other words, the shortest string has to be put as prefix and as suffix of the reverse of the longest.
Strings a and b may be empty, but not null (In C# strings may also be null. Treat them as if they are empty.).
If a and b have the same length treat a as the longer producing b+reverse(a)+b