How should I practice Index of the First Non-Repeating Character?

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Index of the First Non-Repeating Character

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Problem statement

Given a non-empty string s, return the zero-based index of the first character whose total frequency in the string is exactly one.

If no such character exists, return -1. Character comparisons are case-sensitive.

Function

firstNonRepeatingIndex(s: String) → int

Examples

Example 1

s = "swiss"return = 1

The character s repeats, while w occurs once at index 1.

Example 2

s = "aabbc"return = 4

Only c has frequency one, and its index is 4.

Example 3

s = "aabb"return = -1

Every character repeats, so there is no qualifying index.

Constraints

1 ≤ s.length ≤ 100000.
s contains printable ASCII characters.