Find index of first unique character

Q: Find index of first unique character

This question evaluates a candidate's skills in string processing, character frequency analysis, and handling character-encoding edge cases such as case sensitivity, Unicode normalization, and grapheme clusters.

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Question

Given a string s, return the index of the first character that appears exactly once; if none exists, return -1. Assume s may include both lowercase and uppercase English letters. Clarify whether the check is case-sensitive and implement both case-sensitive and case-insensitive variants. Follow-ups: (

Optimize for a small fixed alphabet—can you use constant extra space? (
How would you support full Unicode, including grapheme clusters and normalization issues? (
If s arrives as a stream too large for memory, how would you detect the first unique character online? Analyze time and space complexity.

Find index of first unique character

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