Given a partially filled 9 x 9 Sudoku board , decide whether the currently filled cells are valid. Each cell is a single-character string: a digit 1 – 9 , or . for an empty cell. The board is valid when: every row contains no repeated digit, every column contains no repeated digit, and each of the nine 3 x 3 sub-boxes contains no repeated digit. Only the filled cells are checked — the board does not have to be solvable. Input format Input is [board] , a single 9 x 9 array of single-character strings, e.g. [[[ 5 , 3 , . , ...], ...]] .