36 - Valid Sudoku\.

Medium

Determine if a 9 x 9 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 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

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: Same as Example 1, 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.

Constraints:

• board.length == 9

• board[i].length == 9

• board[i][j] is a digit 1-9 or '.'.