Next in the Alphabet

Create a function which returns the next letters alphabetically in a given string. If the last letter is a "Z", change the rest of the letters accordingly.

Examples

next_letters("A") ➞ "B"

next_letters("ABC") ➞ "ABD"

next_letters("Z") ➞ "AA"

next_letters("CAZ") ➞ "CBA"

next_letters("") ➞ "A"

Notes

• Tests will all be in CAPITALS.
• Empty inputs should return a capital "A" (as if it were in letter position 0!).
• Think about the letter "Z" like the number 9 and how it carries over to increment the next letter/digit over.