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Find First Prefix-Matching Word | Pinterest Coding Question

Quick Overview

This question evaluates proficiency in string processing and prefix-matching within arrays, testing the ability to identify the first occurrence and handle cases such as empty inputs or no matches.

medium
Pinterest
Coding & Algorithms
Software Engineer

Find First Prefix-Matching Word

Company: Pinterest

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: medium

Interview Round: Technical Screen

Given an array of strings `words` and a prefix string `prefix`, return the smallest index `i` such that `words[i]` starts with `prefix`. If no word matches, return `-1`. Example: - `words = ["a", "apple", "appz", "b"]` - `prefix = "ap"` - Output: `1` Explanation: `"apple"` is the first word in the array that starts with `"ap"`.

Quick Answer: This question evaluates proficiency in string processing and prefix-matching within arrays, testing the ability to identify the first occurrence and handle cases such as empty inputs or no matches.

Given an array of strings `words` and a string `prefix`, return the smallest index `i` such that `words[i]` starts with `prefix`. If no such word exists, return `-1`. If multiple words match, you must return the earliest one in the array.

Constraints

0 <= len(words) <= 100000
1 <= len(words[i]) <= 100
1 <= len(prefix) <= 100
All strings may contain lowercase English letters

Examples

Input: (["a", "apple", "appz", "b"], "ap")

Expected Output: 1

Explanation: "apple" is the first word that starts with "ap".

Input: (["hello", "world"], "wor")

Expected Output: 1

Explanation: "world" starts with "wor", and it appears at index 1.

Input: (["dog", "cat", "fish"], "bir")

Expected Output: -1

Explanation: No word in the array starts with "bir".

Input: ([], "a")

Expected Output: -1

Explanation: The array is empty, so there cannot be any matching word.

Input: (["solo"], "solo")

Expected Output: 0

Explanation: The only word exactly matches the prefix, so the answer is index 0.

Hints

You only need the first matching word, so scan the array from left to right.
Use a direct prefix check on each word instead of comparing entire strings.

Last updated: Apr 19, 2026