Measure Rainwater Held Between Bars
Company: Amazon
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: hard
Interview Round: Other
### Problem
The array `heights` describes a row of adjacent vertical bars on level ground. Bar `i` has height `heights[i]`, and every bar is one unit wide.
Return the total volume of rainwater that remains between the bars after rainfall. Water that can escape past either end of the row is not retained. Measure the result in unit squares of the side profile.
### Examples
**Example 1**
```text
Input: heights = [4,0,2,0,4]
Output: 10
```
The three interior positions hold `4`, `2`, and `4` units of water, respectively.
**Example 2**
```text
Input: heights = [1,3,2,4]
Output: 1
```
Only the bar of height `2` has space above it that is enclosed on both sides. That space holds one unit.
### Constraints
- `1 <= heights.length <= 2 * 10^4`
- `0 <= heights[i] <= 10^5`
Overview: Calculate the total rainwater retained between unit-width bars from an elevation array, including separate basins and water that escapes at the ends.
