# https://www.codewars.com/kata/52bc74d4ac05d0945d00054e
#
# Write a function named `first_non_repeating_letter` that takes a string input,
# and returns the first character that is not repeated anywhere in the string.
#
# For example, if given the input `'stress'`, the function should return `'t'`,
# since the letter *t* only occurs once in the string, and occurs first in the
# string.
#
# As an added challenge, upper- and lowercase letters are considered the **same
# character**, but the function should return the correct case for the initial
# letter.  For example, the input `'sTreSS'` should return `'T'`.
#
# If a string contains *all repeating characters*, it should return an empty
# string (`""`) or `None` -- see sample tests.


def first_non_repeating_letter(string):
    s = string.lower()
    for i in range(len(s)):
        if s[i] not in s[:i] and s[i] not in s[i+1:]:
            return string[i]
    return ''
