Section 1Understand Problem
Section 01
Understand the problem
Restate the problem precisely. Interviewers reward candidates who pause, define the contract, and name the guarantees before coding.
Problem statement
Determine if a 9 x 9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules: each row must contain the digits 1-9 without repetition; each column must contain the digits 1-9 without repetition; each of the nine 3 x 3 sub-boxes must contain the digits 1-9 without repetition. The Sudoku board could be partially filled, where empty cells are filled with the character `.`.
Example
board: standard valid example → true
Constraints
board.length == 9 · board[i].length == 9 · board[i][j] is a digit 1-9 or '.'
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