# LeetCode 125: Valid Palindrome — Step-by-Step Visual Trace

**Easy** — Two Pointers | String | Palindrome

## The Problem

Determine if a given string is a valid palindrome, considering only alphanumeric characters and ignoring cases. A palindrome reads the same forward and backward.

## Approach

Use two pointers starting from both ends of the string, moving inward while skipping non-alphanumeric characters. Compare characters in lowercase at each position until pointers meet or a mismatch is found.

**Time:** O(n) · **Space:** O(1)

## Code

```python
class Solution:
    def isPalindrome(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
```

## Watch It Run

> **[Open interactive visualization](https://tracelit.dev/app?trace=0125_valid-palindrome)**

> **Try it yourself:** Open [TraceLit](https://tracelit.dev/app?trace=0125_valid-palindrome) and step through every line.

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*Built with [TraceLit](https://tracelit.dev) — the visual algorithm tracer for LeetCode practice.*