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What is Trace?

Trace runs Python code line-by-line in your browser (via Pyodide / WebAssembly) and visualizes every variable mutation — heaps as trees, linked lists as chains, grids as cells, sets/dicts/queues as native shapes. Perfect for understanding LeetCode solutions, teaching algorithms, or debugging tricky logic.

Supported Input Formats

• nums = [1,2,3], target = 9 — standard LeetCode format
• head = [1,2,3], pos = 1 — auto-builds linked lists (with cycles)
• root = [3,9,20,null,null,15,7] — auto-builds binary trees
• ["MedianFinder", "addNum", ...] / [[], [1], ...] — Design problems (auto-detected)
• Paste directly from LeetCode — Input: prefix handled automatically

Auto-Save & Persistence

Your code and test input are saved to your browser's local storage automatically as you type. Reload the page anytime — your work is preserved. Use Save Snippet to keep a named copy of any solution you want to revisit.

Sharing

Click Share to generate a URL that encodes your code + input. Anyone with the link can open the same scenario in their browser — no account required.

Tips

• The bar at the very top shows your playback progress through all snapshots.
• Variables that changed since the last step are highlighted in green.
• Heap visualizations show raw stored values — for negated max-heaps, you'll see negative numbers.
• For OOP problems, instance attributes (self.x) are auto-captured and shown.
• If execution hangs, the runtime caps at 1000 steps to protect against infinite loops.

Give this snippet a memorable name. It will be saved in your browser only.

Tip: existing snippet with the same name will be overwritten.

Copy this URL — anyone who opens it will see exactly your code & test input.

The URL contains your code (encoded). It works without any backend or account.

Something not working, or a wrong visualization? Tell us what happened — your current code & test input are attached automatically so we can reproduce it.

We attach: your code, test input, current step, theme, and browser info. No personal data.