The cheapest bill

Problem

On Saturday at the market, the fruit seller offers you an odd deal: you tell him how many kilos of each fruit you are taking, he gives you a list of prices per kilo, and you decide which price goes with which fruit. You want to pay as little as you can.

Write a function that takes quantities and prices, two lists of positive integers of the same length (they may be empty), and returns an integer: the lowest total possible. Each quantity is multiplied by one price, and each price is used exactly once.

With quantities 2 and 5 and prices 10 and 4: 5 kilos at 4 and 2 at 10 make 40; the other way, 5 at 10 and 2 at 4 make 58. The answer is 40. With empty lists you pay nothing: you return 0.

Examples

• The example

  [2, 5], [10, 4] → 40

• Three fruits

  [2, 5, 3], [4, 10, 7] → 61

• The heaviest with the cheapest

  [1, 8], [2, 9] → 25

• A single fruit

  [4], [6] → 24

• Equal prices

  [3, 1], [5, 5] → 20

Besides these, the challenge has hidden tests that are revealed when you submit your solution.

You start with this

def bill(quantities, prices):
    pass

function bill(quantities, prices) {
}