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

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

Try it yourself: Open TraceLit and step through every line.

Built with TraceLit — the visual algorithm tracer for LeetCode practice.

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