Elevator, Floor, Request classes in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the role of the Request class in elevator systems

In an elevator system design, what is the primary responsibility of the Request class?

A To represent a user's call for the elevator from a specific floor and direction

B To manage the list of floors serviced by the elevator

C To control the movement of the elevator between floors

D To monitor the elevator's current position and speed

💡 Hint

Think about what a user does when they want to use an elevator.

The Request class models a user's call to the elevator, specifying the floor and direction they want to go. It does not control movement or track elevator status.

❓ Architecture

intermediate

Identifying the correct relationship between Elevator and Floor classes

Which of the following best describes the relationship between the Elevator and Floor classes in a typical elevator system design?

A Elevator contains multiple Floor objects representing floors it can visit

B Floor contains multiple Elevator objects representing elevators stopping there

C Elevator and Floor are independent with no direct association

D Floor inherits from Elevator to reuse movement logic

💡 Hint

Consider how an elevator knows which floors it can go to.

The Elevator class typically maintains a list or reference to Floor objects it can visit. Floors do not contain elevators, and inheritance is not appropriate here.

❓ scaling

advanced

Scaling elevator request handling for a high-rise building

In a high-rise building with 50 floors and 10 elevators, which design choice best improves the scalability of handling elevator requests?

A Let each elevator handle requests independently without coordination

B Use a centralized request queue shared by all elevators

C Assign each elevator its own request queue and coordinate via a scheduler

D Process requests only when elevators are idle to reduce complexity

💡 Hint

Think about balancing load and avoiding conflicts among elevators.

Assigning each elevator its own queue and coordinating via a scheduler allows better load distribution and scalability. Centralized queues can become bottlenecks, and no coordination leads to inefficiency.

❓ tradeoff

advanced

Choosing between polling and event-driven request handling

For the Elevator system's Request class, which tradeoff best describes using an event-driven approach over polling to handle requests?

A Polling reduces latency but uses more memory

B Event-driven reduces CPU usage but increases system complexity

C Event-driven increases latency but simplifies code

D Polling eliminates the need for synchronization mechanisms

💡 Hint

Consider how the system reacts to new requests and resource usage.

Event-driven systems react immediately to requests, reducing CPU waste from constant checking, but require more complex event handling logic. Polling is simpler but less efficient.

❓ component

expert

Estimating capacity for elevator request queue in peak hours

In a building with 20 floors and 5 elevators, during peak hours, each floor generates on average 10 elevator requests per minute. If each elevator can process 15 requests per minute, what is the minimum size of the request queue to avoid dropping requests?

A At least 200 requests to handle double the incoming rate

B At least 50 requests because elevators can process 75 requests total per minute

C At least 10 requests since elevators process requests continuously

D At least 100 requests to buffer all incoming requests

💡 Hint

Calculate total incoming requests per minute and total processing capacity.

Total incoming requests = 20 floors * 10 requests = 200 per minute. Total processing capacity = 5 elevators * 15 requests = 75 per minute. The queue must buffer the difference (200 - 75 = 125) plus some margin. At least 100 is a reasonable minimum to avoid dropping requests.