# Pastebin BTv4Qj5R
def find_the_added_letter_junior(s, t):
    char_count = {}
    for c in t:
        if c in char_count:
            char_count[c] += 1
        else:
            char_count[c] = 1

    for c in s:
        if c in char_count:
            char_count[c] -= 1
        else:
            return c

    for c in char_count:
        if char_count[c] == 1:
            return c

def find_the_added_letter_senior(s, t):
    counter_s = collections.Counter(s)
    counter_t = collections.Counter(t)
    diff = counter_t - counter_s
    return next(iter(diff))

def find_the_added_letter_ascii(s, t):
    char_count = 256 * [0]
    for c in t:
        char_count[ord(c)] += 1
    for c in s:
        char_count[ord(c)] -= 1
    for idx, c in enumerate(char_count):
        if c == 1:
            return chr(idx)


import collections
import random
import string
import timeit

n = 5_000#_000

s = ''.join(random.choices(string.ascii_uppercase + string.digits, k=n))
pos = random.randint (0, n)
t = ''.join([s[:pos], "X", s[pos:]])

print("Junior", timeit.timeit("find_the_added_letter_junior(s, t)", globals=globals(), number=1000))
print("Senior", timeit.timeit("find_the_added_letter_senior(s, t)", globals=globals(), number=1000))
print("Ascii", timeit.timeit("find_the_added_letter_ascii(s, t)", globals=globals(), number=1000))