Constructor

ATMSystem()

• Creates an ATM with no accounts and no cash.
• The ATM initially has no active card.

Add an Account

boolean addAccount(String cardNumber, String pin, int openingBalance)

• Adds an account identified by cardNumber.
• pin must contain exactly four digits.
• Returns true when the account is added.
• Returns false if the card number already exists or any parameter is invalid.

Load Cash

boolean loadCash(int denomination, int count)

• Adds count notes of the given denomination to the ATM.
• Repeated calls add to the existing number of notes.
• Returns true when the notes are added.
• Returns false if the denomination is unsupported or count is not positive.

Customer Operations

Insert a Card

boolean insertCard(String cardNumber)

• Returns true and starts a session when the card exists, is not blocked, and no other card is active.
• Returns false otherwise.
• A newly inserted card is not authenticated until the correct PIN is entered.

Enter a PIN

boolean enterPin(String pin)

• Returns true and authenticates the active card when the PIN is correct.
• Returns false when there is no active card or the PIN is incorrect.
• An incorrect PIN increases the failed-attempt count for that card.
• After three consecutive incorrect attempts, the card is permanently blocked and automatically ejected.
• Failed attempts are counted across sessions for the same card.
• A successful PIN entry resets the card's failed-attempt count to zero.
• If the current session is already authenticated, this method returns false and makes no changes.

Check Balance

int getBalance()

• Returns the balance of the authenticated account.
• Returns -1 if there is no authenticated session.

Withdraw Cash

List<Integer> withdrawCash(int amount)

• Withdraws exactly amount from the authenticated account.
• The amount must be positive and divisible by 100.
• The account must have enough balance, and the ATM must be able to dispense the exact amount using its available notes.
• The ATM uses the smallest possible number of notes.
• If multiple selections use the same number of notes, prefer more notes of higher denominations in this order: 2,000, 500, 200, 100.
• Returns the dispensed note values in descending order.
• Returns List.of(-1) if the withdrawal cannot be completed, the amount is not positive, or the amount is not divisible by 100.
• A failed withdrawal does not change the account balance or ATM cash.

Deposit Cash

boolean depositCash(List<Integer> notes)

• Deposits the supplied notes into the authenticated account.
• Every value in notes must be a supported denomination.
• Returns true after adding the total value to the account balance and adding the notes to the ATM.
• Returns false if there is no authenticated session, the list is empty, or any note is invalid.
• Returns false and makes no changes if the resulting account balance would exceed 1,000,000.
• A failed deposit does not change the account balance or ATM cash.

Cancel the Session

boolean cancelSession()

• Cancels the current session, clears authentication, and ejects the active card.
• It can be called before or after the card is authenticated.
• Returns true when an active session is cancelled.
• Returns false when no card is active.
• It does not reverse any deposit or withdrawal that already completed successfully.
• It does not reset failed PIN attempts for the card.

Eject the Card

boolean ejectCard()

• Ejects the active card, clears authentication, and ends the current session.
• Returns true when a card is ejected.
• Returns false when no card is active.
• It does not reset failed PIN attempts for the card.

Method Call Order

• enterPin returns false if no card is active.
• enterPin returns false and makes no changes if the current session is already authenticated.
• getBalance returns -1 if the active card has not been authenticated.
• withdrawCash returns List.of(-1) if the active card has not been authenticated.
• depositCash returns false if the active card has not been authenticated.
• insertCard returns false if another card is already active.
• cancelSession and ejectCard return false if no card is active.
• After a session is cancelled or a card is ejected, customer operations fail until another card is inserted and authenticated.
• A method called in the wrong order makes no changes, except that an incorrect PIN entered before authentication increases the card's failed-attempt count.

Rules

• Card numbers are case-sensitive.
• Account balances and ATM note counts must never become negative.
• An account balance must never exceed 1,000,000.
• Deposit, withdrawal, and balance operations require an authenticated session.
• Every deposit and withdrawal is completed as one atomic operation.
• A failed operation must not make partial changes.
• Calls are processed sequentially in the order received.
• Deposited notes are immediately available for later withdrawals.
• Cancelling or ejecting a card does not reset its failed PIN attempts.
• Cancelling a session cannot undo an operation that has already completed.
• Administrative methods may be called at any time because all calls are processed sequentially.

Constraints

• 1 ≤ cardNumber.length() ≤ 20
• cardNumber contains only uppercase English letters and digits.
• pin.length() == 4
• pin contains only digits.
• 0 ≤ openingBalance ≤ 1,000,000
• 1 ≤ amount ≤ 1,000,000
• denomination is one of 100, 200, 500, or 2,000.
• 1 ≤ count ≤ 1,000,000
• 0 ≤ notes.size() ≤ 1,000
• At most 100,000 method calls will be made.

Examples

Example 1: Deposit and Withdrawal

Example 2: Card Blocking

Example 3: Failed Withdrawal

Example 4: Cancel a Session

Example 5: Deposit Exceeds the Balance Limit