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Zeni Study

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- Learn Python In 7 Days
  Day 1 : Variable + if/else
  Day 2: Loops
  Day 3 : Array + String
  Day 4 — Functions
  Day 5 — Dictionaries - Hashmap
  Day 6 — Files & Errors
  Day 7 — Mini Project & Programmer Thinking
- Machine Coding:
  Day 1 — Bank account + Wallet :
  Day 2 : RateLimiter + seat booking
  Day 3 : Library Management + Classroom Attendance System
  Day 4 : Bank account and notification system.
  Day 5 : Key-value and payment processor.
  Day 6 : Discount engine + Seat booking( in depth)
  Day 7: Parking lot System :
- Machine coding extended:
  Day 7.5 — Parking Lot (Constraint-Heavy Variant)
  Day 8 : Order + Payment System
  Day 8 — Extension Round (Mid-Interview Change)
  Day 9 — Ticket Booking with Expiry (Simulated Time)
  Day 9 — Extension Round (Seat Lock Ownership + Forced Unlock)
- SQL + DB Thinking course
  Day 1 — Tables, Rows & Thinking in Data
  Day 2 — Relationships & Foreign Keys (Thinking in Connections)
  Day 3 — Filtering, Aggregation & Answering Data Questions
  Day 4 — JOINs (Combining Tables the Right Way)
  Day 5 — Subqueries, EXISTS & NOT EXISTS (Thinking in Layers)
  Day 6 — Indexes, Constraints & How Databases Think
  Day 7 — Full SQL Interview Simulation (SDE-1 Ready)
  Extended Day 1 — Core Business Systems (Deep Practice)
  Extended Day 2 — Booking & Platform Systems (High-Depth Practice)
  Extended Day 3 — Content & Learning Platforms (Capstone Practice)
- LLD: Java
  DAY 1 — Core Foundations (SDE-2 Level)
  DAY 2 — State, Time & Domain Correctness
  DAY 3 — Extensibility & Failure Handling (SDE-2 Core Signal)
  DAY 4 — Concurrency, Scheduling & Consistency (SDE-2 Critical)
  DAY 5 — Workflow Design & Config-Driven Systems (SDE-2+)
  DAY 6 — Eventing & Usage-Based Systems (Advanced SDE-2)
  DAY 7 — Interview Simulation (Hard, Production-Grade)
  Hidden Edge Cases Interviewers Expect (Day-wise)
  Detailed Testing file:

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DAY 1 — Core Foundations (SDE-2 Level)

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🟢 DAY 1 — Core Foundations (SDE-2 Level)

Concepts Introduced Today

You’ll practice:

Time-based logic

Thread safety

Clear API design

Edge-case driven coding

No advanced patterns yet — focus is correctness + clarity.

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✅ Problem 1: Rate Limiter (Sliding Window, Single Node)

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🔹 New Concept (Short Explanation)

Sliding Window Rate Limiter

Instead of fixed buckets:

Track timestamps of requests

Allow request if count in last windowSize ≤ limit

Example:

Limit = 3 requests / 10 seconds

Requests at times:

t=1 -> allowed

t=2 -> allowed

t=3 -> allowed

t=9 -> ❌ blocked (3 already in window)

t=11 -> allowed (t=1 expired)

Key challenges:

Time handling

Removing expired entries

Thread safety

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📌 Problem Statement

Design an in-memory rate limiter.

Requirements

Per-user rate limiting

Sliding window algorithm

Thread-safe

Millisecond precision

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Example

limit = 2 requests / 5 seconds

request("user1") -> true

request("user1") -> true

request("user1") -> false

(wait 5 seconds)

request("user1") -> true

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🧩 Java Interface (DO NOT MODIFY)

public interface RateLimiter {

boolean allowRequest(String userId);

}

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🧪 Driver Code (Auto Tests + Edge Cases)

Java93 lines

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🎯 What Interviewer Evaluates

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