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Grokking the Coding Interview Patterns in C++

Solution: Valid Sudoku

Explore how to validate a 9x9 Sudoku board by implementing a solution that uses hash maps to ensure no digit repeats in any row, column, or 3x3 sub-box. Understand constraint-checking techniques that avoid backtracking while maintaining constant time and space complexity.

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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= 9 ...