请判定一个数独是否有效

public class Solution {
    /**
     * @param board: the board
     * @return: whether the Sudoku is valid
     */
    public boolean isValidSudoku(char[][] board) {
        // write your code here
        for(int i = 0; i < board.length; i++){
            for(int j = 0; j < board[i].length; j++){
                char mark = board[i][j];
                for(int k = j+1; k < board[i].length; k++){
                    if(mark != '.' && board[i][k] != '.' && mark == board[i][k]){
                        return false;
                    }
                }
                char mark1 = board[j][i];
                for(int k = j+1; k < board.length; k++){
                    if(mark1 != '.' && board[k][i] != '.' && mark1 == board[k][i]){
                        return false;
                    }
                }
            }
            char mark2 = board[i][board[i].length - 1];
            for(int j = board[i].length - 2; j >= 0; j--){
                if(mark2 != '.' && board[i][j] != '.' && mark2 == board[i][j]){
                        return false;
                }
            }
            char mark3 = board[i][i];
            for(int j = i + 1; j < board.length; j++){
                if(mark3 != '.' && board[j][j] != '.' && mark2 == board[j][j]){
                        return false;
                }
            }
            if(i == 0 || i == 3 || i == 6 ){
                for(int j = 0; j < 3; j++){
                    List<Character> data = new ArrayList<>(); 
                    for(int m = i; m < i + 3; m++){
                        for(int k = j * 3; k < 3 + j * 3; k++){
                            char mark4 = board[m][k];
                            data.add(mark4);
                        }
                    }
                    for(int m = 0; m < data.size(); m++){
                        char mark = data.get(m);
                        for(int n = m + 1; n < data.size(); n++){
                            if(mark != '.' && data.get(n) != '.' && mark == data.get(n)){
                                return false;
                            }
                        }
                    }
                }
            }
        }
        
        return true;
    }
}

解题思路：
先判断横纵轴的是否存在相同数，再判断交互轴是否存在相同数，最后判断每个小9宫格是否存在相同数。