Problem

Spotify wants a small utility to quickly analyze short text tokens. Write a Python function that returns the index of the first character in a string that appears exactly once.

Formal Specification

Implement a function first_unique_char(s) where:

• Input: a string s containing lowercase English letters
• Output: an integer representing the index of the first non-repeating character
• Return -1 if every character appears more than once

Examples

Example 1

• Input: s = "leetcode"
• Output: 0
• Explanation: 'l' appears once and is the first such character.

Example 2

• Input: s = "loveleetcode"
• Output: 2
• Explanation: 'v' is the first character whose frequency is 1.

Example 3

• Input: s = "aabb"
• Output: -1
• Explanation: Every character appears more than once.

Constraints

• 1 <= len(s) <= 10^5
• s contains only lowercase English letters

Notes

A clean solution should avoid comparing every character with every other character. Aim for a linear-time approach using a frequency count, then scan the string again to find the first index whose count is 1.