Basics: count with a dict, then look up

Write first_unique(s: str) -> int that returns the index of the first character in s that occurs exactly once in the whole string, or -1 if every character repeats.

first_unique("swiss")    # 1   ('w'; 's' repeats, and 'i' comes later)
first_unique("abab")     # -1
first_unique("z")        # 0

• 0 <= len(s) <= 10^5; s can contain any characters, and upper and lower case are different characters.
• Calling s.count(ch) for each position is O(n²) in general. Do it in two passes instead: count, then scan.