Trapping Rain Water LeetCode

trappingRainWater.js

/*
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
0 <= n <= 3 * 104
0 <= height[i] <= 105
*/

var trap = function(height) {
    let result = 0;
    let start = 0;
    let end = height.length - 1;
    while(start < end) {
        if (height[start] <= height[end]) {
            let current = height[start];
            while(height[++start] < current) {
                result += current - height[start];
            }
        } else {
            let current = height[end];
            while(height[--end] < current) {
                result += current - height[end]
            }
        }
    }
    return result;
};

const height = [0,1,0,2,1,0,1,3,2,1,2,1];
trap(height);