# Patterns That Recur in LLD — High-Level & Low-Level Design Interview Problems

Source: https://www.skillbyai.com/en/design-interviews/lld-patterns

> Strategy, state, observer, factory.

## Four patterns and when to use them

**Strategy** swaps an algorithm behind an interface: pricing, elevator scheduling, spot allocation. **State** moves behaviour into one class per state, so a vending machine or order behaves differently in `Idle` versus `HasMoney` without big switch statements. **Observer** lets subscribers react to events, such as display boards updated when spots free up. **Factory** centralises object creation, for example creating the right `Vehicle` or `Spot` from a type. The Design Patterns course covers these in depth; in interviews, name a pattern only after showing the problem it solves.

## Observer for availability updates

Python sketch.

```python
from abc import ABC, abstractmethod

class AvailabilityListener(ABC):
    @abstractmethod
    def on_change(self, floor: int, free_spots: int) -> None: ...

class DisplayBoard(AvailabilityListener):
    def on_change(self, floor, free_spots):
        print(f"Floor {floor}: {free_spots} free")

class Floor:
    def __init__(self, number: int):
        self.number = number
        self.free = 0
        self._listeners: list[AvailabilityListener] = []

    def subscribe(self, listener: AvailabilityListener) -> None:
        self._listeners.append(listener)

    def set_free(self, count: int) -> None:
        self.free = count
        for listener in self._listeners:
            listener.on_change(self.number, count)
```

## Patterns as vocabulary

Patterns are like chess openings: knowing the names lets you communicate quickly, but you still have to play the position in front of you.

**Quiz:** A vending machine that behaves differently when idle, holding money or dispensing is a natural fit for:

- [ ] A bigger switch statement in every method
- [ ] The Singleton pattern
- [ ] The Adapter pattern
- [x] The State pattern

*Answer:* The State pattern. One class per state.