Solution: Valid Sudoku

Understand how to validate a 9x9 Sudoku board by checking rows, columns, and sub-boxes for duplicate digits using hash maps. This lesson demonstrates a fixed-time, single-pass constraint check without backtracking or search, helping you solve similar constraint problems efficiently.

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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 $= 9$ ...