Valid Sudoku - Medium Python Arrays & Hashing Interview Problem

Determine if a 9x9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules:

Each row must contain the digits 1-9 without repetition.
Each column must contain the digits 1-9 without repetition.
Each of the nine 3x3 sub-boxes of the grid must contain the digits 1-9 without repetition.

Note: A Sudoku board does not need to be solvable. Only the currently filled cells need to be validated.

Example 1

[ ["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"] ]

Input: board = (9x9 grid above)
Output: True
Explanation: Each row, column, and 3x3 box contains distinct digits 1-9 (ignoring empty cells ".").

Example 2

[ ["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"] ]

Input: board = (9x9 grid above, "8" appears twice in column 0)
Output: False
Explanation: The digit "8" appears twice in the first column (rows 0 and 3), violating the column rule.

Constraints

board.length == 9
board[i].length == 9
board[i][j] is a digit 1-9 or ".".

Hints