1var assert = require('assert');
2
3function trappingRainWater(heights) {
4  let totalArea = 0;
5  let maxFromLeft = 0;
6  let maxAreaFromLeft = 0;
7
8  for (let height of heights) {
9    totalArea += height;
10    maxFromLeft = Math.max(maxFromLeft, height);
11    maxAreaFromLeft += maxFromLeft;
12  }
13
14  let maxFromRight = 0;
15  let maxAreaFromRight = 0;
16
17  for (let i = heights.length - 1; i >= 0; i--) {
18    maxFromRight = Math.max(maxFromRight, heights[i]);
19    maxAreaFromRight += maxFromRight;
20  }
21
22  const boundingArea = heights.length * maxFromLeft;
23  const leftGap = boundingArea - maxAreaFromLeft;
24  const rightGap = boundingArea - maxAreaFromRight;
25  return boundingArea - leftGap - rightGap - totalArea;
26}
27
28try {
29  assert.equal(
30    trappingRainWater([0, 0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1, 0, 0]),
31    6
32  );
33
34  console.log(
35    'PASSED: `trappingRainWater([0,0,1,0,2,1,0,1,3,2,1,2,1,0,0])` should return `6`'
36  );
37} catch (err) {
38  console.log(err);
39}
40
41try {
42  assert.equal(
43    trappingRainWater([0, 0, 1, 0, 2, 4, 4, 4, 2, 3, 1, 0, 2, 4, 3, 1, 0, 1]),
44    14
45  );
46
47  console.log(
48    'PASSED: `trappingRainWater([0,0,1,0,2,4,4,4,2,3,1,0,2,4,3,1,0,1])` should return `14`'
49  );
50} catch (err) {
51  console.log(err);
52}
53
54try {
55  assert.equal(trappingRainWater([0, 0, 1, 2, 4, 4, 4, 3, 1, 0, 0, 0]), 0);
56
57  console.log(
58    'PASSED: `trappingRainWater([0,0,1,2,4,4,4,3,1,0,0,0])` should return `0`'
59  );
60} catch (err) {
61  console.log(err);
62}