Problem

At Spotify, a common text-processing task is to quickly identify the first character in a string that appears exactly once. Given a string s, return the index of the first non-repeating character. If no such character exists, return -1.

Formal Specification

• Input: A string s
• Output: An integer representing the index of the first unique character, or -1 if none exists

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 that appears exactly once.

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
• The solution should run efficiently for large inputs

A correct solution should avoid comparing every character against every other character. Aim for a linear-time approach using an auxiliary data structure for counting.