> ## 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.

# Valid Palindrome Python Solution with Tests

> Tested Python solution for LeetCode 125 with 16 pytest cases. Generate a practice environment with lcpy.

LeetCode 125, Easy. Topics: Two Pointers, String. [View on LeetCode](https://leetcode.com/problems/valid-palindrome/description/).

Generate this problem as a practice environment: tested reference solution, 16 [parametrized pytest cases](/practice/testing), and a playground notebook:

```bash theme={"theme":{"light":"github-light","dark":"github-dark"}}
lcpy gen -n 125   # by problem number
lcpy gen -s valid_palindrome   # by problem name
```

## Problem

A phrase is a **palindrome** if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.

Given a string `s`, return `true` if it is a **palindrome**, or `false` otherwise.

### Examples

```
Input: s = "A man, a plan, a canal: Panama"
Output: true
```

**Explanation:** "amanaplanacanalpanama" is a palindrome.

```
Input: s = "race a car"
Output: false
```

**Explanation:** "raceacar" is not a palindrome.

```
Input: s = " "
Output: true
```

**Explanation:** s is an empty string "" after removing non-alphanumeric characters. Since an empty string reads the same forward and backward, it is a palindrome.

### Constraints

* `1 <= s.length <= 2 * 10^5`
* `s` consists only of printable ASCII characters.

## Solution

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

```python theme={"theme":{"light":"github-light","dark":"github-dark"}}
class Solution:
    # Time: O(n)
    # Space: O(1)
    def is_palindrome(self, s: str) -> bool:
        left, right = 0, len(s) - 1

        while left < right:
            while left < right and not s[left].isalnum():
                left += 1
            while left < right and not s[right].isalnum():
                right -= 1

            if s[left].lower() != s[right].lower():
                return False

            left += 1
            right -= 1

        return True
```

## Complexity

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

## Tags

[Grind 75](/catalog/grind-75), [Grind](/catalog/grind), [Blind 75](/catalog/blind-75), [NeetCode 150](/catalog/neetcode-150), [NeetCode 250](/catalog/neetcode-250), [NeetCode All](/catalog/neetcode).