Valid Palindrome

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

Examples

The brute-force approach

Filter the string down to only alphanumeric characters and lowercase everything. Then reverse that cleaned string and check if it equals the original cleaned string.

cleaned = [c.lower() for c in s if c.isalnum()]
# cleaned is e.g. ['a','m','a','n',...]

return cleaned == cleaned[::-1]    # ← reverse and compare

This approach is O(n) time and O(n) space because you're building the full cleaned list in memory, then building a second reversed copy to compare. It works, but you can do the check without allocating that extra space.

Spotting the pattern

This is a Two Pointers problem. The key question to ask yourself:

Can I check this from both ends at once, skipping non-alphanumeric characters as I go?

Answering that is where it clicks, and it's exactly what the guided walkthrough below builds with you: the pattern reasoning, a progressive hint ladder that never spoils the answer, a row-by-row dry run, the optimized solution, and an in-browser editor to run your code against real test cases.