Problem Recall: a visual LeetCode pattern drill
Valid Palindrome
A visual flashcard drill for FAANG LeetCode problems. See the problem, recognize the pattern, watch the solution animate. No walls of text — the motion is the explanation.
Valid Palindrome
Reads the same forwards and backwards (letters & digits only).
Two Pointers
Converging from both ends, comparing as they meet.
Step through it
All problems by pattern
Two Pointers
Prefix Sum
Monotonic Deque
Arrays & Hashing
Monotonic Stack
Intervals
Heap / Top-K
Backtracking
Recursive Descent
Frequently asked questions
What is Problem Recall?
Problem Recall is a visual flashcard drill for FAANG LeetCode problems. See the problem, recognize the pattern, watch the solution animate. No walls of text — the motion is the explanation.
How does a drill work?
Every problem has three stages: the problem, the pattern and the solution. Step through them with the Back and Next buttons or the ← and → arrow keys; Escape resets to the start.
Which problems are included?
31 LeetCode problems grouped into 11 patterns. They're all listed on this page, each with its own drill.
Do I need an account?
No. There's no sign-up: open any problem and start the drill.
Can I share a single problem?
Yes. Every problem has its own link (/p/ followed by the problem name), so you can bookmark or share it.