Rain between the rocks

Problem

The storm has passed, and puddles are left between the rocks on the shore. The rocks stand in a row, each one is one unit wide, and the list holds their heights (whole numbers from 0 up; it may be empty).

Above each rock the water rises to the lower of two heights: the tallest rock to its left and the tallest rock to its right. Whatever is above the rock is water; if that height is not above the rock, there is none there. The first and last rocks never hold water: they are missing a side.

With [3, 0, 2, 0, 4], the tallest to the left of the middle rocks is 3 and to the right is 4, so the water reaches 3. The first 0 holds 3, the 2 holds 1 and the other 0 holds 3: you return 7.

Return the total water as a whole number. With no rocks, or one or two, it is 0.

Examples

• The example

  [3, 0, 2, 0, 4] → 7

• A long shore

  [0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1] → 6

• The rocks only go up

  [1, 2, 3, 4] → 0

• The right side is the lower one

  [4, 0, 1] → 1

• A single rock

  [5] → 0

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

You start with this

def trapped_water(rocks):
    pass

function trappedWater(rocks) {
}