### Isomorphic Strings With Map

Two strings are called *isomorphic* if there exists a one-to-one mapping from characters in one string to
characters in the other.

For example, "aab" and "xxy" are isomorphic, because the one-to-one mapping `{'a': 'x', 'b': 'y'}`

,
when applied to "aab", yields "xxy".
However, "acb" and "xxy" are *not* isomorphic, because the mapping `{'a': 'x', 'c': 'x', 'b': 'y'}`

is not
one-to-one, because 'x' appears twice as a value.

One way to think about it is that the mapping should allow us to transform back and forth between the two strings. After transforming "aab" to "xxy", we can invert the mapping to undo the transformation. However, after transforming "acb" to "xxy", we cannot undo the transformation.

Create a method `areIsomorphic`

.
It accepts two non-`null`

`String`

s and returns `true`

if the `String`

s are isomorphic, and `false`

otherwise.
If either `String`

is `null`

, throw an `IllegalArgumentException`

.
Also note that you can begin with a simple length check to rule out certain cases.

A natural way to approach this problem is to use a `Map<Character, Character>`

,
and Kotlin's built-in maps are available for this problem.
You may also find sets useful.