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Determine if a 9 x 9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules: 1. Each row must contain the digits 1-9 without repetition. 2. Each column must contain the digits 1-9 without repetition. 3. Each of the nine 3 x 3 sub-boxes of the grid must contain the digits 1-9 without repetition. Note: - A Sudoku board (partially filled) could be valid but is not necessarily solvable. - Only the filled cells need to be validated according to the mentioned rules.
Example 1
Input: board = [["5","3",".",".","7",".",".",".","."],["6",".",".","1","9","5",".",".","."],[".","9","8",".",".",".",".","6","."],["8",".",".",".","6",".",".",".","3"],["4",".",".","8",".","3",".",".","1"],["7",".",".",".","2",".",".",".","6"],[".","6",".",".",".",".","2","8","."],[".",".",".","4","1","9",".",".","5"],[".",".",".",".","8",".",".","7","9"]]
Output: true
Explanation: No digit repeats within any row, column, or 3x3 box.
Example 2
Input: board = [["8","3",".",".","7",".",".",".","."],["6",".",".","1","9","5",".",".","."],[".","9","8",".",".",".",".","6","."],["8",".",".",".","6",".",".",".","3"],["4",".",".","8",".","3",".",".","1"],["7",".",".",".","2",".",".",".","6"],[".","6",".",".",".",".","2","8","."],[".",".",".","4","1","9",".",".","5"],[".",".",".",".","8",".",".","7","9"]]
Output: false
Explanation: The top-left cell is now 8, but column 0 already has an 8 in row 3, and the top-left 3 x 3 box already has an 8 at (2,2). A digit repeats, so the board is invalid.
Example 3
Input: board = [[".",".",".",".",".",".",".",".","."],[".",".",".",".",".",".",".",".","."],[".",".",".",".",".",".",".",".","."],[".",".",".",".",".",".",".",".","."],[".",".",".",".",".",".",".",".","."],[".",".",".",".",".",".",".",".","."],[".",".",".",".",".",".",".",".","."],[".",".",".",".",".",".",".",".","."],[".",".",".",".",".",".",".",".","."]]
Output: true
Explanation: The board is completely empty, so no digit can repeat and it is valid.
Scan every filled cell once, tracking the digits already seen in each row, each column, and each 3x3 box using nine sets each. If a digit is already present in its row, column, or box, the board is invalid.
Run checks all cases above. Submit evaluates all test cases.