Multiple elevator coordination in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Elevator request assignment strategy

In a building with multiple elevators, which strategy best minimizes average wait time for passengers?

A Assign the request to the elevator that is currently idle on the ground floor.

B Assign the request to the elevator with the fewest passengers regardless of location.

C Assign the request randomly to any elevator to balance load.

D Assign the request to the elevator closest to the pickup floor and moving towards it.

💡 Hint

Think about which elevator can reach the passenger fastest without unnecessary detours.

Assigning the request to the closest elevator moving towards the pickup floor reduces wait time by avoiding unnecessary travel.

❓ Architecture

intermediate

Component responsible for elevator state tracking

Which component in a multiple elevator system is primarily responsible for tracking each elevator's current floor, direction, and door status?

A Elevator Controller Module

B Elevator State Manager

C User Interface Module

D Request Dispatcher Module

💡 Hint

Consider which module holds real-time data about elevator positions and statuses.

The Elevator State Manager maintains real-time information about each elevator's floor, direction, and door status to enable coordination.

❓ scaling

advanced

Scaling elevator coordination for a skyscraper

When scaling a multiple elevator system for a 100-floor skyscraper with 20 elevators, which approach best maintains system responsiveness?

A Divide elevators into groups serving specific floor ranges and coordinate requests within groups.

B Use a single centralized controller to manage all elevators and requests.

C Assign requests randomly to any elevator regardless of floor range.

D Let each elevator operate independently without coordination.

💡 Hint

Think about reducing communication overhead and improving response time by limiting coordination scope.

Grouping elevators by floor ranges reduces complexity and improves responsiveness by localizing coordination.

❓ tradeoff

advanced

Tradeoff between energy efficiency and wait time

In multiple elevator coordination, prioritizing energy efficiency by minimizing elevator movement can increase passenger wait times. Which design choice best balances this tradeoff?

A Always keep elevators idle at the ground floor to save energy.

B Serve requests strictly in the order they arrive without optimization.

C Implement a dynamic scheduling algorithm that adapts based on current traffic patterns.

D Use fixed elevator routes regardless of demand.

💡 Hint

Consider a system that changes behavior based on real-time usage to optimize both goals.

Dynamic scheduling adapts to traffic, balancing energy use and wait times effectively.

❓ estimation

expert

Estimating peak load capacity for multiple elevators

Given a building with 15 elevators and 50 floors, estimate the maximum number of passengers the system can transport per hour during peak times assuming each elevator holds 20 passengers and completes 4 full trips per hour.

A 1200 passengers per hour

B 1500 passengers per hour

C 1000 passengers per hour

D 3000 passengers per hour

💡 Hint

Calculate total capacity by multiplying elevators, capacity per elevator, and trips per hour.

15 elevators × 20 passengers × 4 trips = 1200 passengers per hour.

Practice

1. What is the main goal of multiple elevator coordination in a building?

easy

A. To reduce wait and travel times for passengers

B. To increase the number of elevators in the building

C. To make elevators move randomly

D. To keep all elevators idle at the ground floor

Solution

1. Step 1: Understand elevator coordination purpose

   Multiple elevator coordination aims to improve efficiency by reducing passenger wait and travel times.

2. Step 2: Evaluate options based on goal

   Options B, C, and D do not focus on improving passenger experience or efficiency.

3. Final Answer:

   To reduce wait and travel times for passengers -> Option A

4. Quick Check:

   Goal of coordination = reduce wait/travel times [OK]

Hint: Focus on passenger experience improvement goals [OK]

Common Mistakes:

• Confusing coordination with adding more elevators
• Thinking elevators should stay idle
• Assuming random movement improves service

2. Which of the following is a correct way to assign an elevator to a new request in a multiple elevator system?

easy

A. Always assign the elevator on the ground floor

B. Assign the elevator farthest from the request floor regardless of direction

C. Assign the elevator closest to the request floor moving in the same direction

D. Assign elevators randomly to balance usage

Solution

1. Step 1: Understand assignment criteria

   Elevators should be assigned based on proximity and direction to minimize wait time.

2. Step 2: Analyze options

   Assign the elevator closest to the request floor moving in the same direction matches this logic. Options A, B, and C ignore direction or proximity, causing inefficiency.

3. Final Answer:

   Assign the elevator closest to the request floor moving in the same direction -> Option C

4. Quick Check:

   Closest elevator + direction match = correct assignment [OK]

Hint: Match elevator direction and proximity for assignment [OK]

Common Mistakes:

• Ignoring elevator direction when assigning
• Choosing elevators randomly
• Always picking ground floor elevator

3. Consider a system with 2 elevators: Elevator A at floor 3 moving up with destinations [5, 7], Elevator B at floor 6 moving down with destinations [4, 2]. A request comes from floor 4 to go up. Which elevator should be assigned?

medium

A. Neither elevator

B. Elevator B

C. Either elevator

D. Elevator A

Solution

1. Step 1: Analyze elevator positions and directions

   Elevator A is at floor 3 going up; Elevator B is at floor 6 going down.

2. Step 2: Match request direction and elevator direction

   Request is at floor 4 going up. Elevator A is below floor 4 and moving up, so it can pick up on the way. Elevator B is above floor 4 but moving down, so it cannot pick up going up.

3. Final Answer:

   Elevator A -> Option D

4. Quick Check:

   Elevator moving towards request floor in same direction = Elevator A [OK]

Hint: Pick elevator moving towards request floor in same direction [OK]

Common Mistakes:

• Choosing elevator moving away from request
• Ignoring elevator direction
• Assuming either elevator works

4. In a multiple elevator system, the controller assigns requests but sometimes an elevator gets stuck and does not update its position. What is the likely problem and how to fix it?

medium

A. Elevator state not updated; add regular position updates and health checks

B. Elevator hardware failure; replace elevator immediately

C. Controller assigns requests randomly; fix assignment logic

D. Elevator doors stuck open; fix door sensors

Solution

1. Step 1: Identify cause of stuck elevator in system

   If elevator position is not updated, controller cannot assign requests properly.

2. Step 2: Determine fix

   Adding regular position updates and health checks ensures controller has current elevator status to assign requests correctly.

3. Final Answer:

   Elevator state not updated; add regular position updates and health checks -> Option A

4. Quick Check:

   Missing updates cause stuck state; fix with health checks [OK]

Hint: Ensure elevator regularly reports position to controller [OK]

Common Mistakes:

• Assuming hardware failure without checking software updates
• Blaming random assignment logic
• Ignoring elevator state updates

5. You are designing a multiple elevator coordination system for a 20-floor building with 4 elevators. To minimize average wait time during peak hours, which strategy is best?

hard

A. Assign elevators randomly to requests to balance load

B. Divide floors into zones and assign elevators to zones dynamically

C. Let all elevators serve all floors equally without zoning

D. Keep all elevators idle at ground floor until called

Solution

1. Step 1: Understand peak hour challenges

   High traffic causes many requests; serving all floors equally can cause delays and conflicts.

2. Step 2: Evaluate zoning strategy

   Dividing floors into zones and assigning elevators reduces travel distance and wait time by localizing service.

3. Step 3: Compare other options

   Random assignment or no zoning causes inefficiency; keeping elevators idle wastes capacity.

4. Final Answer:

   Divide floors into zones and assign elevators to zones dynamically -> Option B

5. Quick Check:

   Zoning elevators reduces wait time in tall buildings [OK]

Hint: Use zoning to reduce travel distance and wait time [OK]

Common Mistakes:

• Ignoring zoning benefits in tall buildings
• Assuming random assignment balances load
• Keeping elevators idle wastes capacity