🦄 First Unique Unicorn

Difficulty: Beginner Tags: strings, hash-map, counting Series: CS 102: Problem Solving & Logic

Problem

Find the first character in a string that appears only once, and return its index. If every character repeats, return -1.

Think of it like finding the first unique item in a list - the one that stands out because it appears exactly once!

Input

s = str  # A string to search

Output

int  # Index of first unique character, or -1 if none exist

Constraints

• String length ≤ 100,000
• String contains only lowercase English letters (a-z)

Examples

Example 1: Unique Character Found

Input:  "leetcode"
Output: 0

Why? Let's count each character: - 'l' appears 1 time ← UNIQUE! - 'e' appears 3 times - 't' appears 1 time - 'c' appears 1 time - 'o' appears 1 time - 'd' appears 1 time

The first unique character is 'l' at index 0.

Example 2: No Unique Characters

Input:  "aabb"
Output: -1

Why? Character counts: - 'a' appears 2 times - 'b' appears 2 times

Every character repeats, so there's no unique character. Return -1.

Example 3: Unique Character Later in String

Input:  "loveleetcode"
Output: 2

Why? Character counts: - 'l' appears 2 times - 'o' appears 2 times - 'v' appears 1 time ← UNIQUE! - 'e' appears 4 times - 't' appears 1 time - 'c' appears 1 time - 'd' appears 1 time

The first unique character is 'v' at index 2 (even though 't', 'c', 'd' are also unique).