Time Management in LLD Interviews

Every Low-Level Design interview is a race against time disguised as a design conversation. While the interviewer presents an open-ended problem—"Design an elevator system" or "Design a chess game"—an invisible clock is ticking. The candidate who doesn't understand the structure of this clock will inevitably run out of time before showcasing their best thinking.

This page dissects the anatomy of a typical LLD interview, revealing its hidden phases, evaluation checkpoints, and the rhythm that separates candidates who control the interview from those who are controlled by it.

Regardless of company or interviewer style, virtually every LLD interview follows a four-phase structure. Understanding these phases is critical because what interviewers expect from you changes dramatically as you move between them. A brilliant class diagram presented during the requirements phase signals poor listening skills. An overly detailed requirements discussion when the interviewer expects code suggests inability to execute.

Let's examine each phase in detail:

The time allocations above represent a 45–60 minute interview, which is standard across most companies. Shorter interviews (30–45 minutes) often compress or eliminate the implementation phase, while longer interviews (60–90 minutes) may expand into multiple implementation cycles or design iterations.

Critical insight: These phases are not strictly sequential. Strong candidates create natural transitions, while weak candidates are pushed between phases by interviewers. Controlling phase transitions is a subtle but powerful skill.

The requirements phase is deceptively critical. Candidates often view it as a formality before the "real" design work begins, but interviewers evaluate you heavily during this phase. How you approach ambiguity, what questions you prioritize, and whether you think beyond the obvious—all are visible here.

What interviewers observe:

The requirements phase structure typically unfolds as:

1. Problem Statement (Interviewer provides): "Design a parking lot system" or "Design a library management system"

2. Initial Scoping Questions (You ask 3–5 high-value questions):

   • "What types of vehicles should the parking lot support?"
   • "Is this a single lot or a chain of lots?"
   • "Should we handle payments, or is that out of scope?"

3. Constraint Clarification (Use-case and scale):

   • "How many parking spots should we design for—hundreds, thousands, millions?"
   • "Do we need real-time availability or can it be eventually consistent?"

4. Assumption Documentation (State your assumptions clearly):

   • "I'll assume we're designing for a single multi-story parking lot with 500–1000 spots."
   • "I'll assume payment is out of scope, but we need to issue tickets."

Common mistakes in the requirements phase:

The high-level design phase is where you demonstrate abstraction and modeling skills. You're expected to identify the core entities in the system, establish their relationships, and sketch the overall architecture of your solution. This is conceptual—you're not diving into method signatures yet.

What you should produce in this phase:

The entity identification process:

Experienced candidates use systematic techniques to identify entities:

1. Noun Extraction — Read through user stories and extract nouns. Each noun is a potential entity.

   • "A driver enters the parking lot and receives a ticket" → Driver, ParkingLot, Ticket

2. Entity vs. Attribute Distinction — Not every noun is an entity. License plates are likely attributes of Vehicle, not standalone entities.

3. Hierarchy Recognition — Are there natural hierarchies? Vehicle might have subclasses (Car, Motorcycle, Truck).

4. Behavior Assignment — What behaviors does each entity need? This hints at what should be a full-fledged class vs. a simple value object.

Visualizing relationships:

Even without formal UML, you should sketch relationships:

[ParkingLot] 1 ────────────── * [ParkingSpot]
                                    │
                                    │ 0..1
                                    ▼
                              [Vehicle]
                                    │
                                    │ 1
                                    ▼
                              [Ticket]

This simple diagram communicates:

• A ParkingLot has many ParkingSpots (one-to-many)
• A ParkingSpot may have zero or one Vehicle (optional one-to-one)
• A Vehicle has one Ticket (one-to-one)

Interviewers value candidates who create lightweight visualizations that communicate structure without getting lost in formal diagramming.

The detailed design phase is where abstraction meets precision. You're expected to define class structures, method signatures, and data types. This is where your deep object-oriented knowledge shines—or where gaps become painfully visible.

What you should produce:

Demonstrating depth:

This phase separates candidates with surface knowledge from those with genuine design expertise. Here's an example of shallow vs. deep design:

Shallow:

class ParkingSpot {
    Vehicle vehicle;
    boolean isAvailable();
    void parkVehicle(Vehicle v);
}

Deep:

enum SpotType { COMPACT, REGULAR, LARGE, HANDICAPPED, ELECTRIC }

abstract class ParkingSpot {
    private final String spotId;
    private final SpotType type;
    private final int floor;
    private Vehicle currentVehicle;
    
    protected abstract boolean canFitVehicle(Vehicle vehicle);
    
    public synchronized boolean parkVehicle(Vehicle vehicle) {
        if (!canFitVehicle(vehicle) || currentVehicle != null) {
            return false;
        }
        this.currentVehicle = vehicle;
        return true;
    }
    
    public synchronized boolean removeVehicle() {
        if (currentVehicle == null) return false;
        this.currentVehicle = null;
        return true;
    }
    
    public boolean isAvailable() {
        return currentVehicle == null;
    }
}

The deep version demonstrates:

• Thoughtful attribute design (ID, type, floor)
• Template Method pattern (abstract canFitVehicle for subclasses)
• Thread safety consideration (synchronized methods)
• Proper encapsulation (private fields, public interface)
• Edge case handling (null checks, return values)

Common detailed design evaluation points:

Not all LLD interviews require writing code, but many do. When implementation is expected, you're being evaluated on design-to-code translation fluency—can you turn your beautiful class diagram into working, clean code?

The implementation phase tests:

Strategies for effective implementation:

1. Start with Interfaces/Abstractions — Define the contracts first. This establishes structure and makes the interviewer confident you won't lose architectural integrity.

2. Implement Core Behaviors First — Don't start with getters/setters. Implement the most important method—the one that demonstrates your design works.

3. Acknowledge Skipped Details — Say "I'll skip the boilerplate getter/setter here and focus on the core logic" to show awareness without wasting time.

4. Test Mentally — As you write, walk through a simple scenario: "When a car arrives, we call parkVehicle(), which finds an available spot via findSpot(), marks it occupied..." This catches logical errors early.

What to implement when time is limited:

If you only have 10–15 minutes for implementation, prioritize:

1. The entry point class — The main facade or controller that orchestrates the system
2. One complete hierarchy — If you designed a class hierarchy, implement at least the abstract base and one concrete subclass
3. One complex method — The method with actual logic (e.g., finding an available spot, matching rider to driver)

Explicitly tell the interviewer: "Given time constraints, I'll implement the ParkingLot facade and the core findAvailableSpot() logic. Shall I prioritize differently?"

While the four-phase structure is broadly universal, companies emphasize different phases and have distinct expectations. Understanding these variations helps you calibrate your approach.

Major tech company patterns:

Adapting to interviewer style:

Within any company, individual interviewers have preferences:

• The Questioner — Interrupts frequently with "what if" scenarios. Embrace this—they're helping you explore the design space.

• The Silent Observer — Provides minimal feedback until the end. Check in periodically: "Does this direction make sense so far?"

• The Code-First Interviewer — Wants to see code early. Shorten the design phase and get to implementation faster.

• The Design Purist — Values diagrams and abstractions. Spend more time on class diagrams before any code.

Reading your interviewer: In the first 5 minutes, observe whether they:

• Ask follow-up questions (→ they want dialogue)
• Redirect you to the board (→ they want visuals)
• Suggest skipping to "the interesting part" (→ they want depth in specific areas)

While you're focused on solving the problem, the interviewer is running an internal evaluation timeline. Understanding this timeline helps you know what's being assessed and when.

The interviewer's mental checkpoints:

What happens if you fall behind:

If you're still on requirements at minute 15, the interviewer knows you won't finish. They may:

1. Provide constraints — "Let's assume X and move forward" (they're helping you)
2. Ask targeted questions — "What would the main classes be?" (they're pushing you forward)
3. Reduce scope — "Let's just focus on the parking mechanism" (salvaging the interview)

Recognize these signals and adjust. If the interviewer is narrowing scope, don't fight it—accept the help and demonstrate depth within the reduced scope.

Understanding the typical LLD interview structure transforms uncertainty into a navigable map. Here's what we've learned:

What's next:

Now that you understand the structure of LLD interviews, the next page focuses on Time Allocation Strategy—specifically, how to budget your time across these phases, what to do when a phase runs long, and techniques for maximizing impact within fixed time constraints.