First Unique Character Index

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Problem

Given a string s, return the index of the first character that appears exactly once. If no such character exists, return -1. Implement the solution in Python using an efficient approach based on character frequency counting.

Constraints

1 <= len(s) <= 10^5
s contains only lowercase English letters
Return -1 if no character appears exactly once

Function Signature

def first_unique_char(s):

Your solutionPython 3

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