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

Refactoring a solution proposed earlier (not having to use extra list/memory). This goes over the string twice. So this takes O(n) too like the original solution.

def first_non_repeated_character(s):
    counts = defaultdict(int)
    for c in s:
        counts[c] += 1
    for c in s:
        if counts[c] == 1:
            return c
    return None