Find First Unique

Problem Overview

• Task: Given a lowercase string, return the 1-based index of the first character that appears exactly once, or -1 if none.
• Input/Output: Input string s (1 ≤ length(s) ≤ 100000), lowercase English letters; output an integer index or -1.
• Context: Models AWS search query optimization using the position of the first unique character.
• Source: Asked in Amazon interviews; a coding interview problem and common interview question.