LLD · Classic

Vending Machine

Design a vending machine that accepts coins and notes, dispenses products, and returns correct change from limited coin storage.

Simple summary: Customer picks a product, pays, machine checks stock and payment, dispenses item, then returns change if needed. Example: Coke costs ₹40; customer inserts ₹50 → machine dispenses Coke and returns ₹10 change.

1. Clarify requirements

Fixed product slots with price and quantity.
Accept coins/notes (or card — usually out of scope).
Reject payment if product sold out or underpaid.
Return change using available coin inventory.
Admin can restock products and refill change.
Out of scope: remote monitoring, multiple machines networked.

2. Core entities

Product — id, name, price, quantityInSlot.
Inventory — map of productId → Product.
CoinStore — map of denomination → count (for change).
VendingMachine — state, currentInsertedAmount, select/dispense APIs.
Transaction — product, paid, changeReturned (audit log).

3. State machine

IDLE → (select product) → AWAITING_PAYMENT AWAITING_PAYMENT → (insert money) → check total if total >= price and product available → DISPENSING if cancel → refund inserted → IDLE DISPENSING → decrement stock, return change → IDLE

4. Key operations

selectProduct(code): if inventory[code].qty == 0: return SOLD_OUT selected = code; state = AWAITING_PAYMENT insertMoney(amount): inserted += amount if inserted >= price(selected): dispense(selected) change = inserted - price returnChange(change) inserted = 0; state = IDLE

5. Change-making

Use greedy algorithm on standard coin set (works for canonical systems like INR/USD coins). If exact change impossible, fail safely and ask admin — do not dispense product without returning change.

returnChange(amount): for coin in coins sorted descending: use = min(amount / coin, coinStore[coin]) dispense use coins; amount -= use * coin if amount != 0: raise CANNOT_MAKE_CHANGE

6. Concurrency

One customer at a time on a physical machine — lock the whole purchase flow. For software simulation, synchronize inventory and coinStore updates.

7. End-to-end walkthrough

Customer selects slot B2 (Coke, ₹40, qty 5).
Inserts ₹20, then ₹20, then ₹10 → total ₹50.
Machine decrements Coke qty to 4, dispenses product.
Change ₹10 returned (one ₹10 coin from CoinStore).

Quick revision

Entities: Product, Inventory, CoinStore, VendingMachine.
Flow: select → pay → verify → dispense → change.
Edge cases: sold out, underpaid, cannot make change.
State machine keeps behaviour clear in interviews.