Improve ATM extensibility by replacing fixed properties with a dynamic list

[sookim-1]

• Read CONTRIBUTING.md
• Only edited files inside source folder (IMPORTANT) and commited them with a meaningful message
• Ran generate-playground.sh, no errors
• Opened playground, it worked fine
• Did not commit the changes caused by generate-playground.sh
• Linked to and/or created issue
• Added a description to PR

📌 Summary

This PR enhances the flexibility of the ATM class by replacing its fixed currency properties (hundred, fifty, twenty, ten) with a dynamic moneyPileList array.

🔍 Problem

• The original ATM implementation had hardcoded properties for each denomination (hundred, fifty, twenty, ten).
• Adding a new denomination required modifying the ATM class.
• The withdrawal process was fixed to start from hundred, making it difficult to change the order dynamically.

✅ Solution

• Introduced a moneyPileList array to hold MoneyPile instances.
• ATM now starts the withdrawal process from the first element of moneyPileList, allowing for a flexible chain configuration.
• Now, adding new denominations or modifying the withdrawal order requires no changes to the ATM class, improving scalability.

🛠 Changes

• Replaced fixed hundred, fifty, twenty, ten properties with moneyPileList: [Withdrawing].
• The withdrawal process starts from moneyPileList.first!, making it adaptable.

🔬 Example Usage

let five = MoneyPile(value: 5, quantity: 10, next: nil)
let ten = MoneyPile(value: 10, quantity: 6, next: five)
let twenty = MoneyPile(value: 20, quantity: 2, next: ten)
let atm = ATM(moneyPileList: [twenty, ten, five]) // Customizable order!
atm.withdraw(amount: 25) // Can withdraw using 1x20 + 1x5

🚀 Benefits

• More maintainable: No need to modify ATM for new denominations.
• More flexible: Users can configure withdrawal order dynamically.
• Backward-compatible: Works with existing MoneyPile implementation.

Would love to hear any feedback! 😊