Problem from LeetCode

Determine if a 9x9 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 9 3x3 sub-boxes of the grid must contain the digits 1-9without repetition

The Sudoku board could be partially filled, where empty cells are filled with the character '.'.

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.
• The given board contain only digits 1-9 and the character '.'.
• The given board size is always 9x9.

Input:

   grid = [
  ['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: Except with the 5 in the top left corner being modified to 8. Since there are two 8's in the top left 3x3 sub-box, it is invalid.