rain.js

function rain(heights) {
    var maxHeight = Math.max.apply(Math, heights), rightSide = heights.length - 1;
    var left, right, newLeft, newRight;
    for (left = 0; heights[left] < maxHeight; left++);
    for (right = rightSide; heights[right] < maxHeight; right--);
    var pooled = 0;
    for (var height = maxHeight; height > 0; height--) {
        for (newLeft = left - 1; newLeft >= 0; newLeft--) {
            if (heights[newLeft] == height) {
                left = newLeft;
            }
        }
        for (newRight = right + 1; newRight <= rightSide; newRight++) {
            if (heights[newRight] == height) {
                right = newRight;
            }
        }
        for (var i = left + 1; i < right; i++) {
            if (heights[i] < height) pooled++;
        }
    }
    console.log(heights, 'pooled', pooled);
    return pooled;
}

// rain([1,4,2,5,1,2,3]);
// rain([1, 2, 3, 2, 1]);

function linear_rain(heights) {
    var right = heights.length - 1, left = 0, pooled = 0, tallest, currentIndex, currentHeight, direction = 1, steps = 0;
    while (left < right) {
        // trim off the ends so that everything in the middle is going to pool
        while ((left < right) && (heights[left] <= heights[left + 1])) { left++; steps++; }
        while ((right > left) && (heights[right] <= heights[right - 1])) { right--; steps++; }
        if (left < right) {
            if (heights[left] < heights[right]) { // move left to right
                direction = 1;
                currentIndex = left;
            } else {
                direction = -1;
                currentIndex = right;
            }
            tallest = heights[currentIndex];
            while (left < right) {
                steps++;
                if (direction == 1) left++;
                else right--;
                currentIndex += direction;
                currentHeight = heights[currentIndex];
                if (currentHeight < tallest) {
                    pooled += tallest - currentHeight;
                } else if (currentHeight > tallest) {
                    tallest = currentHeight;
                    break;
                }
            }
        }
    }
    console.log('pooled', pooled, 'from', heights, 'in', steps, 'steps');
    return pooled;
}

linear_rain([1,4,2,5,1,2,3]);
linear_rain([1, 2, 3, 2, 1]);