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 19
without repetition.
Each column must contain the digits 19
without repetition.
Each of the nine 3 x 3
subboxes of the grid must contain the digits 19
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:
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 =
[["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 subbox, it is invalid.
Constraints:
board.length == 9
board[i].length == 9
board[i][j]
is a digit19
or'.'
.

