Valid Sudoku

Determine whether a 9 x 9 Sudoku board is valid. Only the filled cells must be checked: each row, each column, and each of the nine 3 x 3 sub-boxes must contain no repeated digit. The board may be incomplete. Input format: - 9 lines, each with 9 characters. A digit `1`-`9` is a filled cell and `.` is empty. Print `true` or `false`.

The board is always 9 x 9.
Each cell is a digit 1-9 or the character `.`.

53..7....
6..195...
.98....6.
8...6...3
4..8.3..1
7...2...6
.6....28.
...419..5
....8..79

true

83..7....
6..195...
.98....6.
8...6...3
4..8.3..1
7...2...6
.6....28.
...419..5
....8..79

false