Palindrome Checks Done Right
Compare from both ends efficiently.
Palindrome Checks Done Right is a free Coding Interview Prep lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Coding Interview Prep learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
What Is a Palindrome
A palindrome reads the same forward and backward, like racecar or level. Checking for one is a classic warm-up problem. 🔁
The Slice Trick
Python can reverse a string with the slice s[::-1]. Compare it to the original and you have a one-line check.
s = 'level'
print(s == s[::-1]) # TrueSlice Costs Extra Memory
The slice approach is short but builds a whole reversed copy. For huge strings that extra memory can matter in tight limits.
The Two-Pointer Way
Place one pointer at the start and one at the end. Compare and step them inward. This uses O(1) extra space.
i, j = 0, len(s) - 1
while i < j:
if s[i] != s[j]:
break
i += 1; j -= 1Stop in the Middle
The pointers only need to meet halfway. Once i is not less than j, every pair has matched and the string is a palindrome.
Mismatch Means Stop Early
The two-pointer loop can return False the instant a pair differs. You rarely scan the whole string, which is a nice speedup.
def is_pal(s):
i, j = 0, len(s) - 1
while i < j:
if s[i] != s[j]: return False
i += 1; j -= 1
return TrueBoth Are O(n)
Either way you touch each character at most once, so both methods run in linear time. They differ only in extra memory used.
Normalize First
Many problems ignore case and spaces. Clean the string first by lowercasing and keeping only letters and digits.
raw = 'A man, a plan'
clean = ''.join(c.lower() for c in raw if c.isalnum())Then Check the Clean String
Once normalized, run your usual palindrome test on the cleaned version. This is the standard pattern for sentence palindromes.
print(clean == clean[::-1])Pointers Skip Non-Letters
Instead of cleaning first, two pointers can skip any non-alphanumeric character in place. That avoids building a second string entirely.
Substring Palindromes
Harder tasks ask about palindromic substrings. The core idea is expand around center: grow outward from each position while ends match.
Quick Check
One question on palindrome checks.
Recap
You can check palindromes with a quick slice or with memory-light two pointers, and normalize messy input before testing. 🎉
Frequently asked questions
Is the “Palindrome Checks Done Right” lesson free?
Yes — the full text of “Palindrome Checks Done Right” is free to read here on the web, and the Coding Interview Prep course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Coding Interview Prep course, upgrade to CoddyKit PRO.
What will I learn in “Palindrome Checks Done Right”?
Compare from both ends efficiently. You practise Coding Interview Prep with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Coding Interview Prep?
No prior experience is required. Coding Interview Prep on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Palindrome Checks Done Right” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Coding Interview Prep lesson?
Yes. Every Coding Interview Prep lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Characters, ord & chr Tricks
- Count Letters with a Frequency Table
- Palindrome Checks Done Right
- Split, Strip & Rejoin Words