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Build a mini spreadsheet: cells hold ints or formulas like =A1+B2, evaluate them and detect cycles

The practical build that's secretly a graph problem: formulas are a dependency DAG, evaluation is DFS with memoization, and the three-color cycle trick decides whether A1=B1, B1=A1 crashes you or earns the offer.

Updated Aug 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.

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