Valid palindrome
one step at a time.
Compare the two ends and walk inward; one mismatch settles it.
Free to use. Step forward and back at your pace, and share a link to the exact step you are stuck on.
What you actually watch.
The part a dry run on paper loses: the state, at every step.
Compare the two ends and walk inward; one mismatch settles it. That sentence is easy to agree with and hard to hold in your head on step nine, which is where most people lose the thread.
So the visualiser keeps it in front of you: both indexes walking, and the span they have already ruled out. The line of code being executed is highlighted as the picture changes, so the drawing and the code never drift apart. Step forward, step back, or let it play.
Put your own input in. Valid palindrome on a clean example teaches you the shape; valid palindrome on an empty input, a single element, or one with duplicates teaches you the thing an interviewer asks about.
How it runs, step by step.
The same order the visualiser moves in, so the list and the picture match.
- 1Start one index at each end of the data.
- 2Compare what the two ends hold and decide which one can never improve.
- 3Move that pointer inward, which rules out everything it passed.
- 4Stop when the pointers meet: every possibility has been ruled in or out.
Inputs worth trying.
The cases that teach something. Each one is a preset in the tool, with the reason it is there.
- Odd lengthWatch the middle character: it is never compared.
- Even lengthNo middle character: the pointers cross.
- Fails at the endsThe inside is a palindrome, but the first check already says no.
What it costs.
The two numbers an interviewer will ask for, and what they mean here.
- Time
- O(n)counted as real operations while it runs, not quoted from a table
- Space
- O(1)what the algorithm holds besides the input itself
- Pattern
- Two pointersTwo indexes walk towards each other, each step ruling something out.
Once you can see it, go and use it.
Watching is the first ten minutes. The pattern sticks when you solve with it.
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Practice, interviews and the tools, all free to start. Swipe to see the rest.
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About valid palindrome.
Open it in the app and the answers are easier to see than to read.
Open the visualiserWhat does the valid palindrome visualiser show?
Every step of the algorithm on input you choose: both indexes walking, and the span they have already ruled out, with the line of code being executed highlighted beside it. You can step forward, step back, or let it play. Spelled visualizer or visualiser, it is the same tool.
What is the time complexity of valid palindrome?
O(n) time and O(1) space. The visualiser counts the real operations as it runs, so you can watch the number grow instead of taking the formula on trust.
Can I use my own input?
Yes. Type your own array, string or graph and the algorithm runs on that, including the edge cases that break a solution in an interview.
Is it free?
Yes. The app is free to start, with 100 AI credits when you sign up and 100 every month, and watching an algorithm costs no credits.