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This is a Medium difficulty Coding & Algorithms question, commonly asked during Technical Screen rounds at Snapchat.

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This question is commonly asked for Software Engineer candidates at Snapchat during technical interviews.

Find index of first unique character | Snapchat Coding Question

Quick Overview

Find index of first unique character evaluates algorithm design, data structures, correctness, complexity, edge cases, and implementation details in a realistic interview setting. A strong answer states assumptions, handles edge cases, explains trade-offs, and shows how to validate the result clearly.

Find index of first unique character

Company: Snapchat

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: Medium

Interview Round: Technical Screen

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: ( 1) Optimize for a small fixed alphabet—can you use constant extra space? ( 2) How would you support full Unicode, including grapheme clusters and normalization issues? ( 3) If s arrives as a stream too large for memory, how would you detect the first unique character online? Analyze time and space complexity.

Quick Answer: Find index of first unique character evaluates algorithm design, data structures, correctness, complexity, edge cases, and implementation details in a realistic interview setting. A strong answer states assumptions, handles edge cases, explains trade-offs, and shows how to validate the result clearly.

Given a string `s`, return the index of the first character that appears exactly once in `s`. If no such character exists, return `-1`. The string may include both lowercase and uppercase English letters. For this challenge the check is **case-sensitive**, so `'a'` and `'A'` are treated as different characters. **Examples:** - `s = "leetcode"` -> `0` (the first `'l'` is the first character that appears exactly once) - `s = "loveleetcode"` -> `2` (the `'v'` at index 2 is the first unique character) - `s = "aabb"` -> `-1` (no character appears exactly once) **Follow-ups to discuss (no code required):** 1. For a small fixed alphabet (e.g. 52 ASCII letters), can you achieve O(1) extra space? 2. How would you extend this to full Unicode, accounting for grapheme clusters and normalization (NFC/NFD)? 3. If `s` arrives as a stream too large to fit in memory, how would you detect the first unique character online?

Constraints

1 <= len(s) <= 10^5 (an empty string returns -1)
s consists of lowercase and uppercase English letters
The check is case-sensitive: 'a' and 'A' are distinct

Examples

Input: ("leetcode",)

Expected Output: 0

Explanation: Every character in 'leetcode' except 'e','t','c','o','d'... actually 'l' at index 0 appears only once, so the answer is 0.

Input: ("loveleetcode",)

Expected Output: 2

Explanation: 'l','o','e' repeat; 'v' at index 2 is the first character that appears exactly once.

Input: ("aabb",)

Expected Output: -1

Explanation: Both 'a' and 'b' appear twice, so no character is unique; return -1.

Input: ("",)

Expected Output: -1

Explanation: An empty string has no characters, so return -1.

Input: ("z",)

Expected Output: 0

Explanation: A single character is trivially unique; its index 0 is returned.

Input: ("aA",)

Expected Output: 0

Explanation: Case-sensitive: 'a' and 'A' are different characters, each appearing once, so the first one at index 0 is returned.

Input: ("dddccdbba",)

Expected Output: 8

Explanation: 'd','c','b' all repeat; only the final 'a' at index 8 appears exactly once.

Input: ("abcabd",)

Expected Output: 2

Explanation: 'a' and 'b' repeat; 'c' at index 2 is the first character that appears exactly once.

Hints

First count how many times each character occurs. A hash map (or a fixed-size array indexed by character) works well.
After counting, scan the string left to right and return the index of the first character whose count is exactly 1.
Two linear passes give O(n) time. For a fixed alphabet, the count storage is O(1) extra space.

Quick Overview