Valid Sudoku

You are given a 9 x 9 Sudoku board encoded as int[][] board, where 0 denotes an empty cell (instead of '.') and the integers 1 through 9 represent filled digits. Determine whether the board is valid by checking only the filled cells.

A Sudoku board is valid if:

• each row contains no repeated digit from 1 to 9 (ignoring 0s)
• each column contains no repeated digit from 1 to 9
• each 3 x 3 sub-box contains no repeated digit from 1 to 9

The board does not need to be solvable. You only need to verify that the currently filled cells do not violate the rules.

Input / output

• Input: board: int[][] where board.length == 9 and board[i].length == 9
• Output: boolean — true if the filled cells form a valid Sudoku state, otherwise false

Constraints

• board.length == 9
• board[i].length == 9
• 0 <= board[i][j] <= 9
• 0 means the cell is empty
• Only the filled cells must satisfy the Sudoku rules

Follow-up

Can you solve it in a single pass over the board by tracking used digits for each row, column, and 3 x 3 box as you go?