How do you group anagrams together? | Interview Question

Q: How do you group anagrams together?

**Target Audience:** Beginners preparing for coding interviews — you know Python lists, strings and dicts; nothing else assumed. **Difficulty:** Medium **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 words an interviewer expects. Every output shown was produced by running the code with `python3` (3.13); nothing is typed in by hand. ## 1. Why This Even Matters — A Story First A post office receives a sack of letters with scrambled addresses — "eat", "tea", "ate", "tan", "nat", "bat". The rule is: letters whose addresses use exactly the same letters go to the same house. How do you sort the sack? The slow way: take a letter, compare it with every other letter in the sack, pull out the matches, and repeat. That is a comparison for every pair of letters. The smart way: give each letter a **label** that is the same for all its anagrams, and put it in the pigeonhole with

The problem

Question presented to candidate: "Given a list of strings, group the anagrams together and return the groups in any order — for example ["eat", "tea", "tan", "ate", "nat", "bat"] becomes [["eat", "tea", "ate"], ["tan", "nat"], ["bat"]]. Please solve it in Python and give me the complexity."

What a strong answer should cover:

The idea of a canonical key shared by all anagrams, and a hash map from key to list of words, built in one pass.
Two keys: the sorted letters, O(k log k) per word, and a 26-letter count, O(k) per word; total O(n · k log k) or O(n · k).
O(n · k) space for keys and output, and why the pairwise brute force is O(n² · k).
Python specifics: collections.defaultdict(list); keys must be hashable, so "".join(sorted(w)) or tuple(...), never a list; dict preserves insertion order, so groups come out in first-seen order.
Edge cases: empty list, empty string, same letters with different counts.

Clarifying questions expected:

Only lowercase a–z? Does the output order matter?

Code / implementation expected: Yes — a group_anagrams(words) function in Python using a dict/defaultdict, run on the sample and on edge cases.

strings
hashing

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Solution

Target Audience: Beginners preparing for coding interviews — you know Python lists, strings and dicts; nothing else assumed. Difficulty: Medium 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 words an interviewer expects. Every output shown was produced by running the code with python3 (3.13); nothing is typed in by hand.

1. Why This Even Matters — A Story First

A post office receives a sack of letters with scrambled addresses — "eat", "tea", "ate", "tan", "nat", "bat". The rule is: letters whose addresses use exactly the same letters go to the same house. How do you sort the sack?

The slow way: take a letter, compare it with every other letter in the sack, pull out the matches, and repeat. That is a comparison for every pair of letters.

The smart way: g

The solution is hidden so you can try first

13 min read · 11 sections

Python code

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

PythonGroup anagrams with a sorted-string key on six inputs

Practice

PythonSorted key vs letter-count key: same groups, different cost

Practice

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