Solution: Valid Sudoku

Discover how to verify if a 9x9 Sudoku board is valid by using hash maps to track digits in rows, columns, and 3x3 sub-boxes. Learn to efficiently check constraints in a single pass without solving the puzzle, understanding the core logic of Sudoku validation and implementing a constant time and space solution.

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Statement
Solution
- Time complexity
- Space complexity

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 check whether the Sudoku is solvable; only whether the current filled entries obey these rules.

Note: A partially filled Sudoku board can be valid even if it is not necessarily solvable. You only need to verify that the filled cells adhere to the given rules.

Constraints:

board.length $= 9$
board[i].length $= 9$ ...