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Find the length of the longest increasing subsequence.

Almost everyone reaches the O(n²) DP. The signal interviewers want is the O(n log n) patience-sorting trick, plus the honesty to say the array you build along the way is not itself the answer subsequence.

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

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FEDITOR'S NOTE

The trap in the n log n version is believing the 'tails' array is the LIS. It is not; only its length is correct. Candidates who claim otherwise get handed an input where the tails array is a sequence that never appeared, and watching them realize it is half the test.

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