BFS and DFS
one step at a time.
Same loop, different container: a queue visits the graph in layers, a stack dives down one path before backing up.
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.
Same loop, different container: a queue visits the graph in layers, a stack dives down one path before backing up. 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 frontier spreading, and which nodes are settled for good. 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. BFS and DFS on a clean example teaches you the shape; bfs and dfs 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 from a source and mark it seen.
- 2Take the next node from the frontier: a queue spreads in layers, a stack goes deep.
- 3Look at its neighbours, and add the ones never seen before.
- 4Never revisit a node, so every edge is examined once.
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: mark when taken outMarks a node only when it leaves the queue, so two neighbours can both queue it. In DFS the same idea without the "already seen?" check visits a node twice. Try "With a cycle".
What it costs.
The two numbers an interviewer will ask for, and what they mean here.
- Time
- O(V + E)counted as real operations while it runs, not quoted from a table
- Space
- O(V + E)what the algorithm holds besides the input itself
- Pattern
- GraphsExplore in layers, or go deep, and never visit twice.
Once you can see it, go and use it.
Watching is the first ten minutes. The pattern sticks when you solve with it.
Everything in the app, in one place.
Practice, interviews and the tools, all free to start. Swipe to see the rest.
DSA practice
Problems filed under the pattern that solves them, not a sheet.
352 problemsOpenCompany sets
Practise what one company actually asks, the week before the round.
463 companiesOpenCoding interview
Pick the role, write in a real editor, and answer while you type.
scored reportOpenMock interviews, spoken
You answer out loud, it follows up on what you said.
36 topicsOpenDatabase round
DBMS asked the way a panel asks it: indexes, joins, normal forms.
in Core CSOpenOOP round
Inheritance, polymorphism and the follow-up you never rehearse.
in Core CSOpenSystem design
How real systems are built, from the primitives to the AI stack.
27 chaptersOpenSQL practice
Real schemas, your query, the rows it returns, and an honest verdict.
39 questionsOpenAptitude
Quant, logical, verbal, DI and technical, timed like the real test.
280 questionsOpenOnline assessment
A clock, marks per question and hidden tests, like a company's judge.
37 papersOpenAlgorithm visualiser
Watch it run on your own input, one step at a time.
26 algorithmsOpenAI that helps, not solves
Hints one at a time, AI review before a mentor reads it.
100 free creditsOpenResume builder
Build it inside the app, next to the work you practised.
in the appOpenProgress and readiness
One score, and the axis you are weakest on.
tracked dailyOpen
Free to start: 100 AI credits when you sign up and 100 every month. Start free in the app
About bfs and dfs.
Open it in the app and the answers are easier to see than to read.
Open the visualiserWhat does the bfs and dfs visualiser show?
Every step of the algorithm on input you choose: the frontier spreading, and which nodes are settled for good, 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 bfs and dfs?
O(V + E) time and O(V + E) 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 bfs and dfs code fail on some inputs?
Usually one of the known traps. The visualiser ships the real ones for this algorithm (mark when taken out), 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.