Longest Common Prefix

Longest Common Prefix

What is this problem about?

The Longest Common Prefix interview question asks you to write a function that finds the longest starting sequence of characters shared among an array of strings. If the array is ["flower", "flow", "flight"], the longest common prefix is "fl". If there is no common prefix, or if the array is empty, the function should return an empty string "".

Why is this asked in interviews?

This is one of the most ubiquitous string problems for entry-level and junior engineering roles. It tests basic control flow, array iteration, and character-by-character comparison. Interviewers use it to see if a candidate can write clean, readable code and properly handle edge cases (like empty arrays, strings of varying lengths, or strings that are identical).

Algorithmic pattern used

There are several valid patterns here, but the two most common are Vertical Scanning and Horizontal Scanning.

• Horizontal Scanning: Compare the first two strings to find their common prefix, then compare that prefix with the third string, and so on.
• Vertical Scanning: Look at the first character of every string. If they all match, move to the second character of every string, continuing until a mismatch occurs or you reach the end of one of the strings.

Example explanation

Let's use Vertical Scanning on ["apple", "ape", "april"].

• Index 0: Look at the first character of all strings: 'a', 'a', 'a'. They match. Prefix is now "a".
• Index 1: Look at the second character: 'p', 'p', 'p'. They match. Prefix is now "ap".
• Index 2: Look at the third character: 'p', 'e', 'r'. They do NOT match! Since a mismatch happened at index 2, we stop immediately. The longest common prefix is everything we matched before this point: "ap".

Common mistakes candidates make

A very frequent mistake candidates make is encountering an IndexOutOfBounds exception. If you use Vertical Scanning, you must ensure that your index variable does not exceed the length of the shortest string in the array. Another mistake is overusing heavy data structures like Tries for this specific, simple version of the problem; while a Trie works, it unnecessarily inflates space complexity and development time.

Interview preparation tip

To nail the Longest Common Prefix coding problem, focus on writing the Horizontal Scanning method first, as it is the most intuitive to code. Start by assuming the first string is the prefix, and iteratively shorten it (using a while loop and indexOf or startsWith) as you check it against subsequent strings. It's an elegant, highly readable solution that interviewers appreciate.