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Minimum XOR Sum of Two Arrays — Bitmask DP
Assign each element of nums1 to exactly one element of nums2 to minimize XOR sum. Classic assignment bitmask DP: dp[mask] = min XOR sum.
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Gray Code — Binary XOR Construction
Generate n-bit Gray code sequence where adjacent codes differ by exactly 1 bit. Formula: gray(i) = i ^ (i >> 1).
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Repeated DNA Sequences — Rolling Hash
Find all 10-letter DNA sequences that appear more than once. Use rolling hash (4-bit per char) or set of seen substrings.
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Number of Valid Words for Each Puzzle — Bitmask Subset
Count words that contain puzzle[0] and only letters from puzzle. Encode words as bitmasks, for each puzzle enumerate its subsets containing puzzle[0].
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Maximum Product of Word Lengths — Bitmask Intersection
Find max product of lengths of two words with no common letters. Encode each word as bitmask; two words share letters if AND != 0.
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