How do you find the longest common prefix of an array of strings?

Q: How do you find the longest common prefix of an array of strings?

**Target Audience:** Beginners preparing for coding interviews — no prior algorithms knowledge assumed **Difficulty:** Easy **Language:** Python 3 > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. All code is Python, and every output block in the **Verified** section was copied from a real run of the examples on this page. ## 1. Why This Even Matters — A Story First A librarian is handed a stack of index cards: "interspecies", "interstellar", "interstate". She wants one label for the drawer they go in, and the label should be the longest beginning that **every** card shares. She lines the cards up one above the other, left-aligned, and reads down the first column: i, i, i. Same. Second column: n, n, n. She keeps going until a column disagrees (e, e, a at the seventh letter) or a card runs out of letters. Everything to the left of that column is the label: "inters

The problem

Question presented to candidate: "Write a function that takes a list of strings and returns the longest prefix they all share, or an empty string if there is none. For example, flower, flow and flight give fl. Please write it in Python."

What a strong answer should cover:

Vertical scanning: compare character i of every string, stop at the first mismatch or the first string that runs out; return strs[0][:i].
Edge cases handled up front: empty list, a single string, an empty string in the list.
O(n * (L + 1)) time, bounded by the total characters S; O(1) extra space besides the result.
Alternatives named with their cost: horizontal scanning, sorting and comparing the first and last strings, a trie for many queries.
Python specifics: zip(*strs) stops at the shortest string, min(strs)/max(strs) give the lexicographic extremes without sorting, and os.path.commonprefix is character-based, not path-based.

Clarifying questions expected:

"What should an empty list return?"
"Is it case-sensitive?"

Code / implementation expected: Yes — longest_common_prefix(strs) in Python; a short loop, with the zip idiom as a nice variant.

dsa
strings

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Beginners preparing for coding interviews — no prior algorithms knowledge assumed Difficulty: Easy Language: Python 3

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact word an interviewer expects. All code is Python, and every output block in the Verified section was copied from a real run of the examples on this page.

1. Why This Even Matters — A Story First

A librarian is handed a stack of index cards: "interspecies", "interstellar", "interstate". She wants one label for the drawer they go in, and the label should be the longest beginning that every card shares. She lines the cards up one above the other, left-aligned, and reads down the first column: i, i, i. Same. Second column: n, n, n. She keeps going until a column disagrees (e, e, a at the sevent

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10 min read · 10 sections

Python code

Edit the code and run it here. Each printed line with an expected value becomes a test.

PythonVertical scanning on normal and edge-case inputs

Practice

PythonVertical vs horizontal vs sort-the-ends, counting character comparisons

Practice

Discussion

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