Valid Sudoku

Statement

Given a 9 × 9 Sudoku board, determine whether it is valid. A board is considered valid if all of the following conditions hold (considering only the filled cells):

• Each row contains the digits 1–9 at most once.

• Each column contains the digits 1–9 at most once.

• Each of the nine 3 × 3 sub-boxes contains the digits 1–9 at most once.

You do not need to ...