Trapping Rain Water

Problem Description

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: 6
Explanation: 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

Solution (JavaScript)

/**
 * @param {number[]} height
 * @return {number}
 */
var trap = function(height) {
    let left = 0, right = height.length - 1;
    let maxLeft = 0, maxRight = 0;
    let trap = 0;
    
    while(left <= right){
        if(height[left] <= height[right]){
            if(height[left] < maxLeft){
                trap += maxLeft - height[left];
            } else {
                maxLeft = height[left];
            }
            left ++;
        } else {
            if(height[right] < maxRight){
                trap += maxRight - height[right];
            } else {
                maxRight = height[right];
            }
            right --;
        }
    }
    
    return trap;
};