Valid Sudoku

public class Solution {
    

public boolean isValidSudoku(char[][] board) {

        if (board == null || board.length < 1) {
            return true;
        }

        int row = board.length;
        int col = board[0].length;

        Set<Character> set = null;

        for (int i = 0; i < row; i++) {
            set = new HashSet<Character>();
            for (int j = 0; j < col; j++) {
                char tmp = board[i][j];
                if ( tmp != ‘.‘ && set.contains(tmp)) {
                    return false;
                } else {
                    set.add(tmp);
                }
            }
        }

        for (int i = 0; i < col; i++) {
            set = new HashSet<Character>();
            for (int j = 0; j < row; j++) {
                char tmp = board[j][i];
                if (tmp != ‘.‘ && set.contains(tmp)) {
                    return false;
                }
                set.add(tmp);
            }
        }

        int rowMin = 0;
        int rowMax = 2;
        int colMin = 0;
        int colMax = 2;

        while (rowMax < row && colMax < col) {

            if (!check(board, colMin, colMax, rowMin, rowMax)) {
                return false;
            } 

            int[] tmp = { rowMin, rowMax, colMin, colMax };

            while (colMax < col - 1) {
                colMin += 3;
                colMax += 3;
                if (!check(board, colMin, colMax, rowMin, rowMax)) {
                    return false;
                }
            }

            colMin = tmp[2];
            colMax = tmp[3];
            while (rowMax < row - 1) {
                rowMin += 3;
                rowMax += 3;
                if (!check(board, colMin, colMax, rowMin, rowMax)) {
                    return false;
                }
            }

            rowMin = tmp[0] + 3;
            rowMax = tmp[1] + 3;
            colMin = tmp[2] + 3;
            colMax = tmp[3] + 3;
        }

        return true;

    }

    public boolean check(char board[][], int colMin, int colMax, int rowMin, int rowMax) {
        Set<Character>     set = new HashSet<Character>();
        for (int i = rowMin; i <= rowMax; i++) {
        
            for (int j = colMin; j <= colMax; j++) {
                char tmp = board[i][j];
                if (tmp != ‘.‘ && set.contains(tmp)) {
                    return false;
                } else {
                    set.add(tmp);
                }
            }
        }

        return true;
    }

}