> ## Documentation Index
> Fetch the complete documentation index at: https://leetcode-py.wisl.dev/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> leetcode-py is a Python LeetCode practice environment generator with one CLI: lcpy. It is not a service or platform.
> Each problem is a directory under leetcode/ with README.md, solution.py, test_solution.py, helpers.py, and playground.ipynb. lcpy gen creates them from JSON templates bundled with the package.
> Examples are backed by tests; copy them verbatim.

# Trapping Rain Water Python Solution with Tests

> Tested Python solution for LeetCode 42 with 15 pytest cases. Generate a practice environment with lcpy.

LeetCode 42, Hard. Topics: Array, Two Pointers, Dynamic Programming, Stack, Monotonic Stack. [View on LeetCode](https://leetcode.com/problems/trapping-rain-water/description/).

Generate this problem as a practice environment: tested reference solution, 15 [parametrized pytest cases](/practice/testing), and a playground notebook:

```bash theme={"theme":{"light":"github-light","dark":"github-dark"}}
lcpy gen -n 42   # by problem number
lcpy gen -s trapping_rain_water   # by problem name
```

## Problem

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.

### Examples

![Example 1](https://assets.leetcode.com/uploads/2018/10/22/rainwatertrap.png)

```
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.

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

### Constraints

* `n == height.length`
* `1 <= n <= 2 * 10^4`
* `0 <= height[i] <= 10^5`

## Solution

Reference implementation from [solution.py on GitHub](https://github.com/wislertt/leetcode-py/blob/main/leetcode/trapping_rain_water/solution.py), full suite in [test\_solution.py](https://github.com/wislertt/leetcode-py/blob/main/leetcode/trapping_rain_water/test_solution.py):

```python theme={"theme":{"light":"github-light","dark":"github-dark"}}
class Solution:
    # Time: O(n)
    # Space: O(1)
    def trap(self, height: list[int]) -> int:
        if not height:
            return 0

        left, right = 0, len(height) - 1
        left_max = right_max = water = 0

        while left < right:
            if height[left] < height[right]:
                if height[left] >= left_max:
                    left_max = height[left]
                else:
                    water += left_max - height[left]
                left += 1
            else:
                if height[right] >= right_max:
                    right_max = height[right]
                else:
                    water += right_max - height[right]
                right -= 1

        return water
```

## Complexity

| Time | Space |
| ---- | ----- |
| O(n) | O(1)  |

## Tags

[Grind 75](/catalog/grind-75), [Grind](/catalog/grind), [NeetCode 150](/catalog/neetcode-150), [NeetCode 250](/catalog/neetcode-250), [NeetCode All](/catalog/neetcode), [AlgoMaster 75](/catalog/algo-master-75).