isle_problem.js

const waterVolume = a => {
  const max = Math.max(...a)
  let sum = 0
  for (let i = max; i >0; i--) {
    let started = false, tmpSum = 0
    for (let j = 0; j < a.length; j++) {
      if (a[j] >= i) {
        if (!started) {
          started = true
        } else {
          sum += tmpSum
          tmpSum = 0
        }
      } else if (started) {
         tmpSum++
      }
    }
  }
  return sum
}

console.log(waterVolume([1, 2, 1, 2, 4, 3, 1, 3, 4, 5, 6, 2, 1, 2, 3, 1])) // 10