Find the first un-repeated character in a string

Question

What is the quickest way to find the first character which only appears once in a string?

Answer 1

I read through the answers, but did not see any like mine, I think this answer is very simple and fast, am I wrong?

def first_unique(s):
    repeated = []

    while s:
        if s[0] not in s[1:] and s[0] not in repeated:
            return s[0]
        else:
            repeated.append(s[0])
            s = s[1:]
    return None

test

(first_unique('abdcab') == 'd', first_unique('aabbccdad') == None, first_unique('') == None, first_unique('a') == 'a')