The strongest signal

Problem

The Albatross probe is past Jupiter now, and its signal reaches the base weaker every day. Each minute the dish measures how strong it came in; when the noise wins, the reading is negative. The dish can only record k minutes in a row, so someone has to pick the best stretch.

Write a function that takes the list of readings (whole numbers, which can be negative) and k (a whole number, 1 or more), and returns the largest sum of k readings in a row. With [3, -1, 4, 1, -5] and k = 2, the windows add up to 2, 3, 5 and -4: you return 5. You don't need to add up the whole window at every step: when it moves one place, one reading comes in and one goes out.

If every reading is negative, the best sum is negative too. If the list has fewer than k readings, there is no window: return 0.

Examples

• The example

  [3, -1, 4, 1, -5], 2 → 5

• A window of three

  [2, 1, 5, 1, 3, 2], 3 → 9

• The best one is the first

  [9, 8, 1, 1, 1], 2 → 17

• One minute at a time

  [5, -2, 7], 1 → 7

• The window is the whole list

  [4, -1, 2, 3], 4 → 8

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

You start with this

def best_window(readings, k):
    pass

function bestWindow(readings, k) {
}