Problem

At Spotify, you are given a lowercase string s and need to find the index of the first character that appears exactly once. If no such character exists, return -1.

Formal Specification

• Input: A string s containing only lowercase English letters.
• Output: An integer representing the index of the first non-repeating character, or -1 if every character repeats.

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 with frequency 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

A correct solution should run efficiently for large strings. A common approach is to count character frequencies first, then scan the string again to find the first index whose count is 1.