Group and Bucket with a Map
Group anagrams and similar items.
Group and Bucket with a Map is a free Coding Interview Prep lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Coding Interview Prep learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Grouping Is a Pattern
Many problems ask you to group items that share something. A map from key to bucket turns that into a single clean pass. 🗂️
Pick the Grouping Key
The whole trick is choosing a key that is identical for items in the same group. Get this right and the rest is easy.
Bucket with defaultdict
Use a defaultdict(list) so every new key starts an empty bucket. You append items without ever checking if the key exists.
from collections import defaultdict
buckets = defaultdict(list)The Core Loop
For each item, compute its key and append the item to that key's bucket. One line per element groups everything.
for word in words:
buckets[key_of(word)].append(word)Group Anagrams
Anagrams share their sorted letters, so the sorted string is the perfect group key. Sort once, then bucket by it.
k = ''.join(sorted(word))
buckets[k].append(word)Tuple Keys for Counts
When sorting is slow, a tuple of letter counts works as the key too. Tuples are hashable, so they slot into a dict cleanly.
k = tuple(Counter(word)[c] for c in 'abcdefghijklmnopqrstuvwxyz')Bucket by a Property
Group numbers by remainder, parity, or length just by changing the key. The pattern stays identical across problems.
for n in nums:
buckets[n % 3].append(n)Bucket Sort Idea
When values fit a small range, drop each into an indexed bucket and read them in order. That is near-linear sorting.
for x in nums:
bucket[x].append(x)Collect the Results
After bucketing, the answer is usually the dict's values. Convert them to a list when the judge wants the groups themselves.
result = list(buckets.values())Counting per Group
If you only need group sizes, bucket into a Counter or sum lengths at the end. Pick whatever the question actually asks for.
sizes = {k: len(v) for k, v in buckets.items()}Why Mapping Wins
Grouping with a map is O(n) instead of comparing every pair. The hashed key does the matching work for you.
Quick Check
You want to group words so that anagrams land in the same bucket.
Recap
Group by mapping each item's key into a defaultdict bucket in one O(n) pass. Choose the key well and grouping problems melt away. 🚀
Frequently asked questions
Is the “Group and Bucket with a Map” lesson free?
Yes — the full text of “Group and Bucket with a Map” is free to read here on the web, and the Coding Interview Prep course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Coding Interview Prep course, upgrade to CoddyKit PRO.
What will I learn in “Group and Bucket with a Map”?
Group anagrams and similar items. You practise Coding Interview Prep with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Coding Interview Prep?
No prior experience is required. Coding Interview Prep on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Group and Bucket with a Map” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Coding Interview Prep lesson?
Yes. Every Coding Interview Prep lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Sets for Membership and Dedup
- Dictionaries as Lookup Tables
- Counter and defaultdict in Action
- Group and Bucket with a Map