Using LeetCode Run Code for Faster Debugging

Why debugging on LeetCode can feel slow

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When you submit a solution and see a red cross, you only know that something failed, but you don’t see why. Without immediate feedback you may guess at the problem, resubmit, and waste cycles.

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Thesis: Run Code gives you a fast feedback loop

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The Run Code button lets you execute your code against a custom test case instantly, showing output, runtime and memory usage. This mirrors the judge’s limits while letting you verify logic before you commit a submission.

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How Run Code works under the hood

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• You write or paste your solution in the editor.
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• Choose a language (Python 3, Java 8, C++17, etc.).
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• Press the Run Code button.
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• LeetCode compiles the code on its sandbox servers, runs it with the supplied input, and enforces the same time (typically 2 s) and memory limits as the real judge.
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• The result pane prints the program’s stdout, any runtime error, and the measured runtime/memory.
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Worked example: Two Sum

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1. Open the Two Sum problem.
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2. Select Python 3 and paste a simple solution:
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class Solution:\n    def twoSum(self, nums, target):\n        seen = {}\n        for i, n in enumerate(nums):\n            complement = target - n\n            if complement in seen:\n                return [seen[complement], i]\n            seen[n] = i\n        return []\n

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1. In the “Custom Test Case” area, type the JSON‑style input LeetCode expects for this problem: [2,7,11,15] on the first line and 9 on the second line (or the combined format nums = [2,7,11,15], target = 9 depending on the UI).
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2. Click Run Code.
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3. The output pane shows [0,1], runtime ~1 ms, memory ~10 MB.
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4. Now change the input to an edge case, e.g., an empty array: [] and 0. Run again; you see an empty list [] with no error, confirming the code handles the case.
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5. If you deliberately introduce a bug—forgetting to return a value—you will get a runtime error or wrong output, letting you fix it before submitting.
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Trade‑offs and limitations

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• Passing a custom test case does **not** guarantee acceptance; hidden test cases may still fail.
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• The sandbox enforces a timeout (≈2 s) and memory limit; a solution that works locally with larger limits may appear correct in Run Code but hit a limit on the real judge.
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• Run Code does not show the detailed test‑case breakdown that the Submit button provides after acceptance.
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Actionable closing

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Make Run Code your first step after writing a solution: verify syntax, basic logic, and edge cases with a few hand‑crafted inputs. Once you are confident, use Submit to run the full suite. This habit reduces failed submissions and tightens your feedback loop without leaving the LeetCode interface.