Mid-level track

Mid-level

Interviewing for a mid-level role. Expand pattern coverage, get fluent with the most common system design prompts, know the everyday tradeoffs cold, and have stories that show ownership and scope cuts.

Overall progress

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Core patterns

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Every pattern interviewers reach for at the mid level.

Maintain a window of elements over a sequence; expand/shrink to satisfy a constraint while tracking an aggregate.

Two indices walk a sorted (or paired) structure from different positions to converge on a target.

Fast & Slow Pointers (Floyd's)

Two pointers move at different speeds through a linked structure to detect cycles or find midpoints.

Merge Intervals

Sort intervals by start, then sweep merging overlaps.

Place each number at its index by swapping; O(n) sort when values are a known bounded range like 1..n.

In-Place Reversal of LinkedList

Reverse pointers segment by segment without extra space using prev/curr/next bookkeeping.

Level-order traversal using a queue; emit one level at a time.

Recursive or stack-based depth-first traversal; pre/in/post order.

Maintain a max-heap for the lower half and a min-heap for the upper half of a stream.

Subsets (BFS-style generation)

Iteratively or recursively build all subsets/permutations/combinations by extending prior results.

Modified Binary Search

Binary search on a non-trivial space: rotated arrays, infinite arrays, the answer itself.

Maintain a heap of size k to track the k largest/smallest/most-frequent elements.

Use a min-heap of k pointers (one per list) to merge k sorted sources in O(N log k).

0/1 Knapsack (DP)

Choose/skip decisions over items with capacity; tabulate optimal value for subproblems.

Monotonic Stack/Queue

Stack/deque maintained in increasing or decreasing order to answer next-greater/smaller queries.

Explore a graph by levels (BFS) or branches (DFS), tracking visited nodes.

DFS through a decision tree, undoing choices on failure to try alternatives.

Medium problems across patterns

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Mostly Blind 75 mediums — the most-asked problems at mid-level loops.

Longest Substring Without Repeating Characters

medium · blind75

medium · blind75

Container With Most Water

medium · blind75

Linked List Cycle II

medium · neetcode150

Merge Intervals

medium · blind75

Insert Interval

medium · blind75

Meeting Rooms II

medium · neetcode150

Find the Duplicate Number

medium · neetcode150

Binary Tree Level Order Traversal

medium · blind75

Validate Binary Search Tree

medium · blind75

Lowest Common Ancestor of a BST

medium · blind75

medium · neetcode150

medium · neetcode150

Search in Rotated Sorted Array

medium · blind75

Find Minimum in Rotated Sorted Array

medium · blind75

Top K Frequent Elements

medium · blind75

Kth Largest Element in an Array

medium · blind75

K Closest Points to Origin

medium · neetcode150

medium · blind75

Longest Increasing Subsequence

medium · blind75

Course Schedule

medium · blind75

Number of Islands

medium · blind75

medium · blind75

Implement Trie (Prefix Tree)

medium · blind75

Daily Temperatures

medium · neetcode150

medium · blind75

Combination Sum

medium · neetcode150

Generate Parentheses

medium · neetcode150

System design building blocks

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The concepts you'll point at when sketching basic designs.

Distribute requests across healthy backends (L4 or L7).

Edge-cached static (and sometimes dynamic) content close to users.

Cache-Aside (Lazy Loading)

App reads cache first; on miss, loads from DB and populates cache.

Relational DB (SQL)

ACID, joins, secondary indexes; vertical scale + read replicas.

Key-Value Store

O(1) get/put by key; massively scalable (DynamoDB, Redis, Cassandra).

Partition data across DB instances by key (hash, range, or geography).

Copy data across nodes for availability and read scaling (sync or async).

Decouple producers/consumers; buffer bursts; enable retries (SQS/RabbitMQ).

Fan-out events to many subscribers; topic-based (Kafka, SNS, Redis pub/sub).

Throttle requests per user/IP with token bucket, leaky bucket, or sliding window.

Single entry point: auth, rate limit, routing, transformation.

Persistent bidirectional connection for low-latency push to clients.

Starter SD prompts

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Get fluent on the 3-4 prompts that come up most often before tackling the long tail.

Design a URL shortener supporting ~100M new URLs/day and ~10B reads/day. Short links must be 7 chara

Design a Distributed Rate Limiter

Throttle API requests to N per minute per API key across a fleet of services, with low overhead and

Design a Chat App (WhatsApp/Messenger)

Design 1:1 and group messaging with delivery receipts, online presence, and offline message storage.

Design Twitter / News Feed

Design a Twitter-like timeline: 500M users, average 200 follows, p95 timeline read < 200ms.

Production techniques

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The toolbox you'll reach for in design discussions and code reviews.

Exponential Backoff + Jitter

Grow retry intervals exponentially and add randomization so clients don't synchronize their retries.

Idempotency Keys

Caller sends a unique key with each request; server dedupes on it so retries don't double-charge.

Circuit Breaker

Track failures from a dependency; once they exceed a threshold, open the circuit and fail fast for a cool-down period.

Probabilistic set membership in tiny memory. No false negatives; tunable false-positive rate.

Bucket refills at rate R; each request takes one token. Allows controlled bursts up to bucket size.

Requests enter a queue, processed at fixed rate. Smooths out bursts entirely — no spike ever reaches downstream.

Coalesce many small operations into one large one to amortize fixed per-request overhead.

Wait until a flurry of events stops for N ms, then act once. Drops everything in between.

Emit at most once per N ms regardless of how often it's called. Unlike debounce, never goes silent.

64-bit unique IDs: [timestamp][machine-id][sequence]. Time-ordered, distributed, no central coordinator.

Grant time-bounded exclusive ownership. Holder must renew before expiry; if it dies, the lease auto-releases.

Periodic 'I'm alive' signal. Absence of N consecutive heartbeats means the peer is presumed dead.

Optimistic Concurrency / CAS

Read a value with its version, send update conditional on version unchanged. Server rejects if someone else wrote in the meantime.

Tradeoffs you'll get asked about

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The ones that come up nearly every loop.

PostgreSQL (SQL) vs DynamoDB (Key-Value NoSQL)

You're building a banking ledger with multi-row transactions and complex reports across accounts, tr

Process synchronously in the request handler vs Enqueue and process asynchronously

A user uploads a video that must be transcoded into 5 resolutions before being playable.

Modular monolith vs Microservices

Small team (8 engineers) building a new B2B SaaS product.

Cache-aside vs Write-through cache

Read-heavy workload where staleness for a few seconds is tolerable.

Optimistic locking (version/etag) vs Pessimistic locking (DB row lock)

Multiple users may edit the same document, but concurrent edits are rare.

Kafka vs AWS SQS

You need to fan out user events to ~10 downstream services and let each independently replay the las

REST + JSON vs gRPC + Protobuf

You're designing internal service-to-service APIs across polyglot backends with strict latency SLAs

Long polling vs WebSockets

A collaborative editor needs sub-100ms updates across users.

Scale vertically (bigger box) vs Scale horizontally (more boxes)

Single Postgres instance approaching CPU saturation as traffic grows.

Stateless servers + external store vs Stateful servers with sticky sessions

Designing a fleet of API servers that need to scale horizontally based on traffic.

Behavioral

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Mid-level interviews probe scoping, conflict, basic mentorship, and accountability for shipped work.

Tell me about a time you had to cut scope to hit a deadline.

Prioritization & Tradeoffs

Walk me through a time you missed a significant deadline.

Failure & Ownership

Tell me about a time you strongly disagreed with a peer's technical decision. How did you handle it?

Conflict & Influence

Tell me about a piece of feedback you received that was hard to hear. What did you do with it?

Growth & Feedback

Describe how you've helped a junior or struggling engineer grow.

Leadership & Mentorship

Give an example of a project where the requirements were unclear. How did you proceed?

Ambiguity & Judgment

Tell me about a technical decision you made that turned out to be wrong. What was the impact and what did you learn?

Failure & Ownership

Walk me through a production incident you led the response to.

Crisis & Operations

Big O — full coverage

All complexity classes, with the average-vs-worst nuance for hashmaps, quicksort, etc.

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