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

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

Find first unique character | Amazon Coding Question

Quick Overview

Find 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 first unique character

Company: Amazon

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: Medium

Interview Round: Technical Screen

Given a lowercase/uppercase alphanumeric string s, return the index of the first character that appears exactly once; if none exists, return -1. Provide an O(n) approach and discuss space–time trade-offs against sorting-based solutions.

Quick Answer: Find 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. If no such character exists, return -1. The comparison is case-sensitive (so 'a' and 'A' are different characters), and the string may contain alphanumeric characters. Provide an O(n) approach. Compared to a sorting-based grouping, the hash-count approach preserves original order without storing extra index metadata and runs in O(n) instead of O(n log n). Example 1: Input: s = "leetcode" Output: 0 ('l' is the first character that appears exactly once.) Example 2: Input: s = "loveleetcode" Output: 2 ('v' at index 2 is the first character to appear exactly once.) Example 3: Input: s = "aabb" Output: -1 (Every character repeats, so there is no unique character.)

Constraints

0 <= len(s) <= 10^5
s consists of alphanumeric characters (comparison is case-sensitive).
Return -1 when no character appears exactly once (including the empty string).

Examples

Input: ("leetcode",)

Expected Output: 0

Explanation: 'l' is the first character that appears exactly once, at index 0.

Input: ("loveleetcode",)

Expected Output: 2

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

Input: ("aabb",)

Expected Output: -1

Explanation: Every character repeats, so no unique character exists.

Input: ("",)

Expected Output: -1

Explanation: Empty string edge case: there is no character to return.

Input: ("z",)

Expected Output: 0

Explanation: Single-character string: that character is trivially unique at index 0.

Input: ("aA",)

Expected Output: 0

Explanation: Case-sensitive: 'a' and 'A' are different characters, so 'a' at index 0 is unique.

Input: ("aabbc",)

Expected Output: 4

Explanation: Last-character-unique case: only 'c' appears once, at index 4.

Input: ("abcabd",)

Expected Output: 2

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

Hints

First pass: count how many times each character appears using a hash map.
Second pass: walk the string in order and return the index of the first character whose count is 1.
Scanning the original string (not the map) on the second pass is what preserves the 'first' / original-order requirement.
A sorting-based approach groups equal characters but loses original order unless it also stores indices, and costs O(n log n) versus O(n) here.

Quick Overview