Sales Season

A retailer is having a store-wide "buy 2, get 1 free" sale. For legal reasons, they can't charge their customers $0 for an article so a discount is applied to all products instead. For example, if a customer gets three products a, b and c:

She gets the cheapest one for free, so she ends up paying $15.99 + $23.50 = $39.49, but what her receipt says is:

• Product A: $15.99 − Special Discount = $12.57
• Product B: $23.50 − Special Discount = $18.47
• Product C: $10.75 − Special Discount = $8.45
• Total: $39.49

Create a function that takes in a list of prices for a customer's shopping cart and returns the prices with the discount applied. Round all prices to the cent.

Examples

discount([2.99, 5.75, 3.35, 4.99]) ➞[2.47, 4.74, 2.76, 4.12]
# First product for free.

discount([10.75, 11.68]) ➞ [10.75, 11.68]
# No discounts applied.

discount([68.74, 17.85, 55.99]) ➞ [60.13, 15.62, 48.98]
# Second product for free.

discount([5.75, 14.99, 36.83, 12.15, 25.30, 5.75, 5.75, 5.75]) ➞ [5.16, 13.45, 33.06, 10.91, 22.71, 5.16, 5.16, 5.16]
# First and sixth products for free (see second note).

Notes

• The discount is calculated in percentual terms.
• The deal applies to sets of three products: if a customer gets 9 products, she will get the three cheapest ones for free, but if she gets 10 or 11 products, she will still get three for free. Buying a 12th product would get her a fourth free product.
• No cart splitting allowed.