First Unique Character

L3 Easy Strings

Concept

Some questions need two passes: first gather global knowledge (counts), then walk the input again and act on it.

Given a string s, return the index of the first character that appears exactly once. If every character repeats, return -1.

Examples

▸ s = "swift"

→ 0

▸ s = "aabb"

→ -1

▸ s = "abcaab"

→ 2

Progressive Hints

Hint 1 · Nudge

Two passes: count everyone first, then find the first one with a lonely count.

Hint 2 · Plan

Pass one builds a frequency map of s. Pass two walks s left to right and returns the first index whose character has a count of exactly 1. If you finish without finding one, return -1.

Hint 3 · Approach

counts = frequency map of s. For each i, c in s: if counts[c] == 1, return i. Return -1.

Output

// Run your code to see the output here.