Adjacent Letters
You are given a string, where all letters are of same case. This string could include adjacent letters - two the same letters together ("aa", "bb" etc). Your task in this mission is to remove both these letters. If after removing one pair a new appears - remove it as well! Each such pair should be removed from string until no one remains. Good luck!
Input: String (str).
Output: String (str).
Examples:
assert adjacent_letters("adjacent_letters") == "adjacent_lrs"
assert adjacent_letters("") == ""
assert adjacent_letters("aaa") == "a"
assert adjacent_letters("ABBA") == ""
