Day 83 — Mock: DSA + System Design I

mock day · 60-minute DSA block · 30-minute system-design discussion

Tonight's goal: choose one weakest DSA topic, run one honest timed mock, then explain one system-design topic out loud with the seven-step script.

Tonight's protocol

Use the mock / mixed day protocol, then use the revision-day protocol only for what failed.

0:00–0:05 — Warm up and choose: answer the MCQs below, pick one DSA row that feels least automatic, and pick one system-design topic.
0:05–1:05 — DSA mock: start one strict 60-minute timer. First say the pattern name, brute force, better approach, time, and space out loud; then code with no solution text.
0:30 checkpoint inside DSA: if still stuck 25 minutes into the DSA timer, read only the pattern name from the menu, restart your explanation, and continue.
1:05–1:35 — System design: explain one topic out loud for 30 minutes using requirements → estimate scale → API → data model → high-level boxes → bottleneck → failure modes.
1:35–1:50 — Revise what failed: write one WA: <root cause> line for every miss. If DSA failed, keep that problem in the next revision queue; if system design failed, redraw the weakest box and say the trade-off again.

The problem menu

Pick exactly one DSA problem for the 60-minute mock. Choose the topic that feels least automatic tonight. Explain aloud before typing.

Weakest topic	Problem	Pattern name
Arrays / hashing	Two Sum	Hashing: the seen question
Strings / window	Longest Substring Without Repeating Characters	Sliding Window I: The Shrinking Rule
Stack	Daily Temperatures	Monotonic stack
Linked list	Reorder List	linked-list composition
Trees	Diameter of Binary Tree	Piggyback on Height: tree DFS return values
Heap	Kth Largest Element in an Array	Heap Top-K: min-heap of size k
Graphs	Number of Islands	grid DFS / flood a component
DP	Coin Change	unbounded minimum DP

For the 30-minute system-design discussion, pick one of the seven course topics and run the same seven-step script out loud.

Topic	Discussion focus
System Design I: Scale, Latency, CAP	scale up, scale out, CAP during partition
System Design II: caching	cache hit, cache miss, invalidation
System Design III: Load Balancing	spread traffic across interchangeable servers
System Design IV: database choices	choose storage by query and consistency needs
System Design V: URL shortener	short code, redirect path, database lookup
System Design VI: Rate Limiter	allow, reject, and track request budgets
System Design VII: a chat system	fan-out, persistent connections, delivery state

Warm-up MCQs

1. Which move starts the mock correctly?

2. When does revision count tonight?

3. Which phrase matches scaling basics?

4. During a partition, what trade-off matters?

5. A learner codes silently, then explains after passing. What broke?

6. What enters the next revision queue?

Self-check gate

Pass tonight if all four are true

The DSA attempt started by minute 5 and stopped at the 60-minute mark.
You said the DSA pattern, brute force, better approach, time, and space before coding.
The system-design topic used all seven steps out loud in 30 minutes.
Every miss has one WA: <root cause> line.

Failure path: add the failed DSA problem to the next revision queue. For system design, add the weakest topic link and the missing step, such as WA: skipped failure modes during CAP discussion.