> ## 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.

# Zigzag Conversion Python Solution with Tests

> Tested Python solution for LeetCode 6 with 20 pytest cases. Generate a practice environment with lcpy.

LeetCode 6, Medium. Topics: String. [View on LeetCode](https://leetcode.com/problems/zigzag-conversion/description/).

Generate this problem as a practice environment: tested reference solution, 20 [parametrized pytest cases](/practice/testing), and a playground notebook:

```bash theme={"theme":{"light":"github-light","dark":"github-dark"}}
lcpy gen -n 6   # by problem number
lcpy gen -s zigzag_conversion   # by problem name
```

## Problem

The string `"PAYPALISHIRING"` is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility)

```
P   A   H   N
A P L S I I G
Y   I   R
```

And then read line by line: `"PAHNAPLSIIGYIR"`

Write the code that will take a string and make this conversion given a number of rows:

```
string convert(string s, int numRows);
```

### Examples

```
Input: s = "PAYPALISHIRING", numRows = 3
Output: "PAHNAPLSIIGYIR"
```

```
Input: s = "PAYPALISHIRING", numRows = 4
Output: "PINALSIGYAHRPI"
Explanation:

P     I    N
A   L S  I G
Y A   H R
P     I
```

```
Input: s = "A", numRows = 1
Output: "A"
```

### Constraints

* `1 <= s.length <= 1000`
* `s` consists of English letters (lower-case and upper-case), `','` and `'.'`.
* `1 <= numRows <= 1000`

## Solution

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

```python theme={"theme":{"light":"github-light","dark":"github-dark"}}
class Solution:
    # Time: O(n)
    # Space: O(n)
    def convert(self, s: str, num_rows: int) -> str:
        if num_rows == 1 or num_rows >= len(s):
            return s

        rows: list[list[str]] = [[] for _ in range(num_rows)]
        current_row = 0
        step = 1
        for char in s:
            rows[current_row].append(char)
            if current_row == 0:
                step = 1
            elif current_row == num_rows - 1:
                step = -1
            current_row += step

        return "".join("".join(row) for row in rows)
```

## Complexity

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

## Tags

[NeetCode All](/catalog/neetcode).