How do I pick the data structure?

Start from the slowest operation you cannot afford. Choose the structure that makes it cheap, then check what that choice does to the others.

Do I write the whole class at once?

No. Get one method working against the example, then add the next. Staged requirements reward small steps.

Pattern · 25 problems

Class Design Interview Problems

Design problems ask for a class, not a function. I list the methods and the cost I want from each, then choose structures.

Get the link for your laptop Start with Kth Largest Element in a Stream

Free: write the code and run it, no sign-up.

When I reach for it

The statement describes a class with methods, not one function.
Each method needs to be fast, and the structure decides how fast.
Requirements arrive in stages.

Problems, easiest first

2 easy · 12 medium · 11 hard. Open one to read it, then write it in the free editor.

Kth Largest Element in a StreameasyHeaps Trees
Implement Queue using StackseasyStack
Implement Trie (Prefix Tree)mediumTrees Strings
Design Add and Search Words Data StructuremediumTrees Strings
Encode and Decode StringsmediumStrings
Design TwittermediumLinked Lists Heaps
Detect SquaresmediumArrays & Hashing
LRU CachemediumLinked Lists Arrays & Hashing
Min StackmediumStack
Time Based Key-Value StoremediumBinary Search Arrays & Hashing
Design Memory Allocatormedium
In-Memory SQL Enginemedium
Snack Bin Iterator — Flatten Nested ListsmediumStack
Web CrawlermediumGraphs Strings
Serialize and Deserialize Binary TreehardTrees BFS & DFS
Find Median from Data StreamhardHeaps Two Pointers
Encrypt and Decrypt StringshardStrings Arrays & Hashing
GPU Credits Managerhard
Design Excel Sum FormulahardGraphs Strings
Carnival Concession Inventoryhard
Cloudflare Rate Limiterhard
Feature Flag Timeline Storehard
Figma Document Bufferhard
Pop-up Clinic Check-in Tablehard
Robinhood Order Bookhard

The idea

Write down every method and the cost you want for it. Then pick the structure that delivers those costs, such as a hash map plus a list.

State the invariants. What must stay true after every call? Check them on the error paths too.

from collections import deque

class HitCounter:
    def __init__(self):
        self.hits = deque()          # timestamps, oldest first

    def hit(self, t):
        self.hits.append(t)

    def count(self, t):
        while self.hits and self.hits[0] <= t - 300:
            self.hits.popleft()      # drop what has expired
        return len(self.hits)

A hit counter for the last five minutes. The deque keeps timestamps in order, so expired ones leave from the front. Python shown; the idea carries to every language in the editor.

Where it goes wrong

Choosing a structure before listing what each method must do.
Breaking an invariant on an error path.
Skipping the questions: size limits, duplicates and thread safety.

Say this before you type. Say each method, the cost you want from it, and the structure that earns that cost.

Where Coco comes in

Coco is an AI study partner, not a person. She listens while you explain your code, asks what a line does and never hands you the fix. Sign in free to type with Coco, up to a daily limit, and to talk with her for one hour. After that, talking comes with the $79 pack. Anyone can open the editor and run their code.

Questions people ask

How do I pick the data structure?: Start from the slowest operation you cannot afford. Choose the structure that makes it cheap, then check what that choice does to the others.
Do I write the whole class at once?: No. Get one method working against the example, then add the next. Staged requirements reward small steps.

Meet these next

Fin and Coco are characters, not people. strongyes.io is independent. Leo builds it with AI agents.