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This is a hard 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.

Count unique Morse-code word transformations

Quick Overview

This question evaluates a candidate's ability to implement string transformations based on a fixed character-to-code mapping and to identify distinct results, testing competencies in encoding, string manipulation, and deduplication.

Count unique Morse-code word transformations

Company: Amazon

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: hard

Interview Round: Technical Screen

Given an array of lowercase strings `words`. Each letter 'a' to 'z' has a corresponding Morse code string (standard 26-letter mapping). A word’s **transformation** is the concatenation of the Morse codes of its letters in order. Return the number of **distinct transformations** among all words. Example: if "cab" transforms to "-.-..--..." ("-.-." + ".-" + "-..."), then two words are considered the same if their full transformed strings are identical.

Quick Answer: This question evaluates a candidate's ability to implement string transformations based on a fixed character-to-code mapping and to identify distinct results, testing competencies in encoding, string manipulation, and deduplication.

Given a list of lowercase English words, convert each word into its Morse-code transformation and return how many distinct transformations exist. The Morse mapping is the standard 26-letter mapping in order from 'a' to 'z': ['.-', '-...', '-.-.', '-..', '.', '..-.', '--.', '....', '..', '.---', '-.-', '.-..', '--', '-.', '---', '.--.', '--.-', '.-.', '...', '-', '..-', '...-', '.--', '-..-', '-.--', '--..']. A word's transformation is formed by concatenating the Morse code of each character from left to right. Two words are considered the same if their full transformed strings are identical.

Constraints

0 <= len(words) <= 1000
1 <= len(word) <= 20
Each word contains only lowercase English letters 'a' to 'z'

Examples

Input: ["gin", "zen", "gig", "msg"]

Expected Output: 2

Explanation: The words 'gin' and 'zen' share one transformation, and 'gig' and 'msg' share another, so there are 2 distinct transformations.

Input: ["a"]

Expected Output: 1

Explanation: There is only one word, so there is exactly one transformation.

Input: []

Expected Output: 0

Explanation: With no words, there are no transformations.

Input: ["cab", "abc", "bac", "cab"]

Expected Output: 3

Explanation: The duplicate 'cab' does not increase the count. The transformations of 'cab', 'abc', and 'bac' are all different, so the answer is 3.

Hints

Use a fixed array of 26 Morse strings and map each character with its alphabetical index.
Store each transformed word in a set so duplicates are counted only once.

Last updated: May 19, 2026

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