from collections import defaultdict

def countLetters(word):
    counter = defaultdict(int) # key=str value=int
    for letter in word:
        counter[letter] += 1
    return counter

s = 'Reverse this strings'
s = s[::-1]
print(s)

input = "Welcome, User_12!!"
s = ''.join(filter(str.isalnum, input))
print(s)    # WelcomeUser12