How do you check if a string is a valid palindrome?

Q: How do you check if a string is a valid palindrome?

**Target Audience:** Beginners preparing for coding interviews — no prior algorithms knowledge assumed **Difficulty:** Easy **Language:** Python 3 > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. All code is Python, and every output block in the **Verified** section was copied from a real run of the examples on this page. ## 1. Why This Even Matters — A Story First A sign painter is asked to paint "Was it a car or a cat I saw?" on a wall that people will read from both sides through a glass door. The client asks: does it read the same backwards? The painter does not care about the capital W, the spaces or the question mark; only the letters matter. So she puts one finger on the first letter, one on the last, and walks them towards each other, skipping anything that is not a letter, comparing as she goes. The first time two letters differ, she can stop: it is not

The problem

Question presented to candidate: "Given a string, tell me whether it is a palindrome if you only look at letters and digits and ignore case. For example, "A man, a plan, a canal: Panama" should be true. Please write it in Python."

What a strong answer should cover:

Converging two pointers that skip non-alphanumeric characters and compare case-insensitively; stop at the first mismatch.
The inner skip loops also check left < right, so a string of only punctuation does not run out of bounds.
O(n) time and O(1) extra space, versus O(n) extra space for the clean-and-reverse one-liner.
Edge cases: empty string, only punctuation, single character, digits versus letters ("0P" is false).
Python specifics: str.isalnum() and str.lower() are Unicode-aware; s[::-1] builds a full copy; casefold() is the stricter case-insensitive comparison.

Clarifying questions expected:

"Only letters and digits, and is it case-insensitive?"
"ASCII or full Unicode? And is an empty string a palindrome?"

Code / implementation expected: Yes — is_palindrome(s) with two indices in Python; the one-line cleaned version is a good warm-up to mention.

dsa
two-pointers
strings

Try it yourself

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Solution

Target Audience: Beginners preparing for coding interviews — no prior algorithms knowledge assumed Difficulty: Easy Language: Python 3

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact word an interviewer expects. All code is Python, and every output block in the Verified section was copied from a real run of the examples on this page.

1. Why This Even Matters — A Story First

A sign painter is asked to paint "Was it a car or a cat I saw?" on a wall that people will read from both sides through a glass door. The client asks: does it read the same backwards? The painter does not care about the capital W, the spaces or the question mark; only the letters matter. So she puts one finger on the first letter, one on the last, and walks them towards each other, skipping anything tha

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6 min read · 10 sections

Python code

Edit the code and run it here. Each printed line with an expected value becomes a test.

PythonTwo-pointer palindrome check on tricky inputs

Practice

PythonValid palindrome II (one deletion) and the clean-and-reverse version

Practice

Discussion

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