Vending Machine - Problem

Design a comprehensive Vending Machine system that handles different operational states and coin transactions.

Your vending machine should support the following functionality:

State Management: Handle three states - 'idle', 'coin_inserted', and 'dispensing'
Coin Operations: Accept coins, calculate total inserted amount
Product Selection: Allow product selection and verify sufficient funds
Change Calculation: Return exact change when overpaid
Refunds: Handle refund requests in appropriate states

Implement a VendingMachine class with methods to handle all operations and return appropriate status messages.

The machine should track its current state and reject invalid operations (e.g., selecting product without inserting coins).

Input & Output

Example 1 — Basic Operations

$ Input: operations = [["insert_coin", 25], ["select_product", "coke"], ["get_state"]]

› Output: ["Coin inserted. Total: 25. State: coin_inserted", "Dispensing coke. Change: 0", "Current state: idle, Coins: 0"]

💡 Note: Insert 25 cents → state becomes coin_inserted, select coke (costs 25) → dispense with no change and return to idle state

Example 2 — Insufficient Funds

$ Input: operations = [["insert_coin", 10], ["select_product", "coke"], ["refund"]]

› Output: ["Coin inserted. Total: 10. State: coin_inserted", "Insufficient funds. Need 15 more", "Refunded: 10. State: idle"]

💡 Note: Insert 10 cents, try to buy coke (25 cents) → insufficient funds, then refund the 10 cents

Example 3 — Invalid Operations

$ Input: operations = [["select_product", "chips"], ["refund"], ["insert_coin", 20]]

› Output: ["Please insert coins first", "No coins to refund", "Coin inserted. Total: 20. State: coin_inserted"]

💡 Note: Try to select product without coins → error, try to refund with no coins → error, then insert coins properly

Constraints

1 ≤ operations.length ≤ 100
Valid operations: insert_coin, select_product, refund, get_state
Coin values: 1 ≤ coin ≤ 100
Products: coke (25), chips (15), candy (10)

Visualization

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Asked in

a Amazon 35 M Microsoft 28 G Google 22 A Apple 18

The key insight is implementing a proper state machine with three states: idle, coin_inserted, and dispensing. This ensures operations are validated against the current state and prevents invalid sequences. Best approach uses State Machine with Validation. Time: O(n), Space: O(1)

Common Approaches

✓ Simple Class Without State Tracking

⏱️ Time: O(n) Space: O(1)

Create a basic vending machine class that processes each operation as it comes, without maintaining proper state transitions or validation.

State Machine with Proper Validation

⏱️ Time: O(n) Space: O(1)

Create a robust vending machine with explicit state tracking and validation to ensure operations are only allowed in appropriate states.

Simple Class Without State Tracking — Algorithm Steps

Process each operation immediately
Calculate totals without state validation
Return basic success/failure messages

Visualization

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Step-by-Step Walkthrough

Receive Operation

Accept any operation without validation

Process Immediately

Execute operation without checking current state

Return Result

Return basic success/failure message

Code -

solution.c — C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct {
    int coins;
} VendingMachine;

VendingMachine* createMachine() {
    VendingMachine* machine = malloc(sizeof(VendingMachine));
    machine->coins = 0;
    return machine;
}

char* processOperation(VendingMachine* machine, char* operation, int value, char* product) {
    static char result[200];
    
    if (strcmp(operation, "insert_coin") == 0) {
        machine->coins += value;
        sprintf(result, "Coin inserted. Total: %d", machine->coins);
    } else if (strcmp(operation, "select_product") == 0) {
        int price = 0;
        if (strcmp(product, "coke") == 0) price = 25;
        else if (strcmp(product, "chips") == 0) price = 15;
        else if (strcmp(product, "candy") == 0) price = 10;
        
        if (price > 0) {
            if (machine->coins >= price) {
                int change = machine->coins - price;
                machine->coins = 0;
                sprintf(result, "Dispensing %s. Change: %d", product, change);
            } else {
                sprintf(result, "Insufficient funds. Need %d more", price - machine->coins);
            }
        } else {
            strcpy(result, "Product not available");
        }
    } else if (strcmp(operation, "refund") == 0) {
        int refund = machine->coins;
        machine->coins = 0;
        sprintf(result, "Refunded: %d", refund);
    } else {
        strcpy(result, "Unknown operation");
    }
    
    return result;
}

int main() {
    printf("[]\n");
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n)

Process each operation once

✓ Linear Growth

Space Complexity

O(1)

Only store basic variables like total coins

✓ Linear Space

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Medium Frequency

~25 min Avg. Time

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Vending Machine - Problem

Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Simple Class Without State Tracking — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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