First Unique Character in a String

Make two linear passes. First count character occurrences; second scan from left to right and return the first index whose character count is one. This preserves the word first, unlike iterating a frequency map. For lowercase English letters, a 26-element count array is O(1) extra space; for arbitrary text, use a map keyed by the correct character unit for the language. In JavaScript and UTF-16 languages, clarify whether the problem treats code units or user-visible Unicode code points as characters, because indexing differs for some symbols. Test empty input, one character, all repeated characters, a unique character at the end, and punctuation. The time is O(n) and space is O(k), where k is the alphabet or number of distinct characters; fixed lowercase alphabet makes the extra space effectively O(1).