get-rainwater.js

/*
* What this function does:
* Given n non-negative integers representing an elevation map where the width of each bar is 1,
* compute how much water it is able to trap after raining.
*/

function getRainCapacity(array) {
  var arrayLength = array.length;
  var water = 0;
  /*
  * left contains height of tallest bar up to that point
  * including itself, going from left to right.
  * right contains height of tallest bar up to that point
  * including itself, going from right to left.
  */
  var left = [];
  var right = new Array(arrayLength);

  left[0] = array[0];
  for (var i = 1; i < arrayLength; i++) {
    left[i] = Math.max(left[i-1], array[i]);
  }

  right[arrayLength - 1] = array[arrayLength - 1];
  for (var i = arrayLength - 2; i >= 0; i--) {
    right[i] = Math.max(right[i+1], array[i]);
  }

  /*
  * calculate the amount of water trapped at each point in the array
  */
  for (var i = 1; i < arrayLength; i++) {
    water += Math.min(left[i], right[i]) - array[i];
  }

  return water;
}

var input = [2, 5, 1, 2, 4, 7, 2, 1, 4, 6, 9, 4];
console.log(getRainCapacity(input));