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`

Idea:

Two pointers traversal

Solution:

``````/**
* @param {number[]} height
* @return {number}
*/
var trap = function(height) {
let res = 0;
const n = height.length;
let l = 0;
let r = n - 1;

while (l < r) {
let min = Math.min(height[l], height[r]);

// if left side is smaller
if (min === height[l]) {
l++;
while (l < r && height[l] < min) {
res += min - height[l++];
}
} else { // right side is smaller
r--;
while (l < r && height[r] < min) {
res += min - height[r--];
}
}
}
return res;
};``````