Trapping Rain Water

Problem statement

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining.

Example 1:

Input: height = [0,1,0,2,1,0,1,3,2,1,2,1]Output: 6Explanation: The above elevation map (black section) is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped.

Example 2:

Input: height = [4,2,0,3,2,5]Output: 9

Constraints:

• n == height.length
• 1 <= n <= 2 * 104
• 0 <= height[i] <= 105

My solution

/**
 * @param {number[]} height
 * @return {number}
 */
var trap = function(height) {
    let waterAmount = 0;
    let leftHeight = Array.from({
        length: height.length
    }, () => 0)
    let rightHeight = Array.from({
        length: height.length
    }, () => 0)
    
    leftHeight[0] = height[0];
    rightHeight[height.length - 1] = height[height.length - 1];
    
    for (let index = 1; index < height.length; index++) {
        leftHeight[index] = Math.max(height[index], leftHeight[index - 1])
    }
    
    for (let index = height.length - 2; index >= 0; index--) {
        rightHeight[index] = Math.max(height[index], rightHeight[index + 1])
    }
    
    for (let index = 0; index < height.length; index++) {
        const boundary = Math.min(leftHeight[index], rightHeight[index]);
        
        if (boundary > height[index]) {
            waterAmount += boundary - height[index]
        }
    }
    
    return waterAmount
};