solution.py

# Find the missing letter

# Write a method that takes an array of consecutive (increasing) letters
# as input and that returns the missing letter in the array.

# You will always get an valid array. And it will be always exactly one letter be missing.
# The length of the array will always be at least 2.
# The array will always contain letters in only one case.

# Example:

# ['a','b','c','d','f'] -> 'e'
# ['O','Q','R','S'] -> 'P'
# (Use the English alphabet with 26 letters!)


def find_missing_letter(chars):
    alphabet = [
        'a',
        'b',
        'c',
        'd',
        'e',
        'f',
        'g',
        'h',
        'i',
        'j',
        'k',
        'l',
        'm',
        'n',
        'o',
        'p',
        'q',
        'r',
        's',
        't',
        'u',
        'v',
        'w',
        'x',
        'y',
        'z',
    ]

    is_upper = chars[0].istitle()
    chars = [i.lower() for i in chars]

    sl = alphabet[alphabet.index(chars[0]) : alphabet.index(chars[len(chars) - 1]) + 1]

    diff = list(set(sl) - set(chars))[0]
    return diff.upper() if is_upper else diff