Valid parentheses
one step at a time.
Push every open bracket; each close must match the top, and the stack must end empty.
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.
Push every open bracket; each close must match the top, and the stack must end empty. 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: the stack growing and popping, with the item on top highlighted. 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 parentheses on a clean example teaches you the shape; valid parentheses 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.
- 1Read the input left to right.
- 2Push what is unfinished and still waiting for a partner.
- 3When the current item resolves the top of the stack, pop it and record the answer.
- 4What is left on the stack at the end is what never resolved.
Where people get this one wrong.
These are not made up. Each one is a switch in the tool: turn it on, run the same input, and watch the state go wrong.
- Bug: no final checkReturns true at the end without looking at the stack, so brackets that were opened and never closed slip through. Try "Left open".
Inputs worth trying.
The cases that teach something. Each one is a preset in the tool, with the reason it is there.
- Crossed pairsEvery bracket has a partner of its kind, but the pairs cross.
- Left openEvery close matches, but two opens never close. Run "Bug: no final check" here.
- One bracketRun the no-final-check bug here too.
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(n)what the algorithm holds besides the input itself
- Pattern
- StackThe most recent unfinished thing is the one you need.
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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About valid parentheses.
Open it in the app and the answers are easier to see than to read.
Open the visualiserWhat does the valid parentheses visualiser show?
Every step of the algorithm on input you choose: the stack growing and popping, with the item on top highlighted, 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 parentheses?
O(n) time and O(n) space. The visualiser counts the real operations as it runs, so you can watch the number grow instead of taking the formula on trust.
Why does my valid parentheses code fail on some inputs?
Usually one of the known traps. The visualiser ships the real ones for this algorithm (no final check), so you can switch the bug on, run it, and watch exactly where the state goes wrong.
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.