Description:
Determine if a 9x9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules:

  1. Each row must contain the digits 1-9 without repetition.
  2. Each column must contain the digits 1-9 without repetition.
  3. Each of the 9 3x3 sub-boxes of the grid must contain the digits 1-9 without repetition.


A partially filled sudoku which is valid.

The Sudoku board could be partially filled, where empty cells are filled with the character ‘.’.

Example 1:

Input:
[
  ["5","3",".",".","7",".",".",".","."],
  ["6",".",".","1","9","5",".",".","."],
  [".","9","8",".",".",".",".","6","."],
  ["8",".",".",".","6",".",".",".","3"],
  ["4",".",".","8",".","3",".",".","1"],
  ["7",".",".",".","2",".",".",".","6"],
  [".","6",".",".",".",".","2","8","."],
  [".",".",".","4","1","9",".",".","5"],
  [".",".",".",".","8",".",".","7","9"]
]
Output:

Example 2:

Input:
[
  ["8","3",".",".","7",".",".",".","."],
  ["6",".",".","1","9","5",".",".","."],
  [".","9","8",".",".",".",".","6","."],
  ["8",".",".",".","6",".",".",".","3"],
  ["4",".",".","8",".","3",".",".","1"],
  ["7",".",".",".","2",".",".",".","6"],
  [".","6",".",".",".",".","2","8","."],
  [".",".",".","4","1","9",".",".","5"],
  [".",".",".",".","8",".",".","7","9"]
]
Output:

Explanation: Same as Example 1, except with the 5 in the top left corner being
modified to 8. Since there are two 8’s in the top left 3x3 sub-box, it is invalid.

题意：验证给定的一个九宫格板上已填的数字是否符合既定的规则；

解法：最简单的办法就是按照给定的规则判断每一行、每一列和每个小九宫格是否数字重复；

class Solution {
    public boolean isValidSudoku(char[][] board) {
        for(int i=0; i<9; i++){
            char[] digits = new char[] {'1','2','3','4','5','6','7','8','9'};
            for(int j=0; j<9; j++){
                if(board[i][j] == '.') continue;
                if(digits[board[i][j] - '1'] != '.') digits[board[i][j] - '1'] = '.';
                else return false;
            }
        }//checking each row
        for(int i=0; i<9; i++){
            char[] digits = new char[] {'1','2','3','4','5','6','7','8','9'};
            for(int j=0; j<9; j++){
                if(board[j][i] == '.') continue;
                if(digits[board[j][i] - '1'] != '.') digits[board[j][i] - '1'] = '.';
                else return false;
            }
        }//checking each column
        for(int k=0; k<9; k++){
            int rowSt = k/3*3;
            int rowEd = rowSt + 3;
            int colSt = k%3*3;
            int colEd = colSt + 3;
            char[] digits = new char[] {'1','2','3','4','5','6','7','8','9'};
            for(int i=rowSt; i<rowEd; i++){
                for(int j=colSt; j<colEd; j++){
                    if(board[i][j] == '.') continue;
                    if(digits[board[i][j] - '1'] != '.') digits[board[i][j] - '1'] = '.';
                    else return false;
                }
            }
        }//checking each sub-boxes
        return true;
    }
}