Methods

• ShoppingCart(List<String> items)
  - Each row in items is a comma-separated string: itemId,pricePerUnit,unitsAvailable.
  - Example: coca-cola-pack,8,12 means 12 units of coca-cola-pack available, each costing 8 dollars.
  - itemId contains only a-z and hyphen -.
  - Each itemId is non-blank and unique.
  - pricePerUnit and unitsAvailable are positive integers < 1000.
  
• String addItem(String itemId, int count)
  - count is always a positive integer.
  - If itemId doesn't exist in catalog ⇒ return UNAVAILABLE.
  - If itemId exists but has less than count units available ⇒ return OUT OF STOCK.
  - If item is correctly added ⇒ return SUCCESS.
  - If an item is already in the cart, simply add count to its existing units.
• List<String> viewCart()
  - Returns current cart items sorted by itemId (lexicographically).
  - Each row is itemId,itemCount.
  - If cart is empty, return an empty list.
• int checkout()
  - Compute total cost as sum of pricePerUnit * itemCount for all items in cart.
  - Clear the cart and return this total.
  - after checkout, the initial items count is not reset. - e.g. if out of 12 coca-cola-pack 3 were ordered then after checkout ,
  9 coca-cola-pack units will remain, they will not reset to 12 again. - If cart is empty, return -1.

Example Scenarios

Example 1 – Normal Flow

ShoppingCart cart = new ShoppingCart(Arrays.asList(
    "coca-cola-pack,8,12",
    "juice-box,5,10"
));

String r1 = cart.addItem("coca-cola-pack", 2); // "SUCCESS"
String r2 = cart.addItem("juice-box", 3);      // "SUCCESS"

List<String> view = cart.viewCart();
// view = ["coca-cola-pack,2", "juice-box,3"]  (sorted by itemId)

int total = cart.checkout();
// total = 2*8 + 3*5 = 31
// cart is now empty; cart.viewCart() returns []

Example 2 – Out of Stock

ShoppingCart cart = new ShoppingCart(Arrays.asList(
    "laptop,900,1"
));

String r1 = cart.addItem("laptop", 2);
// r1 = "OUT OF STOCK"
// cart.viewCart() = []  (still empty)

Example 3 – Unavailable Item & Empty Checkout

ShoppingCart cart = new ShoppingCart(Arrays.asList(
    "book,50,2"
));

String r1 = cart.addItem("pen", 1);
// r1 = "UNAVAILABLE"
// cart.viewCart() = []  (cart still empty)

int total = cart.checkout();
// total = -1  (cannot checkout empty cart)