Valid Sudoku
Determine if a 9x9 Sudoku board is valid Rules: Each row, column, and 3x3 box must contain 1-9 without repetition Empty cells are represented by '.'
Valid Sudoku looks straightforward, but it quietly checks whether you reach for the right lookup structure. Interviewers love it because it reveals your data-structure instincts and your edge-case discipline. The classic trap is mishandling duplicates or overwriting useful information too early.
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