36. 有效的数独(java)

判断一个 9x9 的数独是否有效。只需要根据以下规则，验证已经填入的数字是否有效即可。

数字 1-9 在每一行只能出现一次。
数字 1-9 在每一列只能出现一次。
数字 1-9 在每一个以粗实线分隔的 3x3 宫内只能出现一次。


上图是一个部分填充的有效的数独。

数独部分空格内已填入了数字，空白格用 '.' 表示。

示例 1:

输入:
[
  ["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"]
]
输出: true
示例 2:

输入:
[
  ["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"]
]
输出: false
解释: 除了第一行的第一个数字从 5 改为 8 以外，空格内其他数字均与 示例1 相同。
     但由于位于左上角的 3x3 宫内有两个 8 存在, 因此这个数独是无效的。
说明:

一个有效的数独（部分已被填充）不一定是可解的。
只需要根据以上规则，验证已经填入的数字是否有效即可。
给定数独序列只包含数字 1-9 和字符 '.' 。
给定数独永远是 9x9 形式的。

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/valid-sudoku
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

解法一：暴力

class Solution {
    public boolean isValidSudoku(char[][] board) {
        for(int i = 0; i < 9; i++){
            int[] count = new int[9];
            for(int j = 0; j < 9; j++){
                if(board[i][j] == '.') continue;
                int n = board[i][j]-'1';
                if(count[n]==0) count[n]++;
                else return false;
            }
        }
        for(int i = 0; i < 9; i++){
            int[] count = new int[9];
            for(int j = 0; j < 9; j++){
                if(board[j][i] == '.') continue;
                int n = board[j][i]-'1';
                if(count[n]==0) count[n]++;
                else return false;
            }
        }
        for(int i = 0; i < 9; i++){
            int[] count = new int[9];
            for(int j = 0; j < 9; j++){
                int a = (i/3)*3+j/3;
                int b = (i%3)*3+j%3;
                System.out.println(a+"|"+b);
                if(board[a][b] == '.') continue;
                int n = board[a][b]-'1';
                if(count[n]==0) count[n]++;
                else return false;
            }
        }        
        return true;
    }
}

解法二：数组模拟哈希

class Solution {
  public boolean isValidSudoku(char[][] board) {
    // init data
      int[][] rows = new int[9][9];
      int[][] columns = new int[9][9];
      int[][] boxes = new int[9][9];

    // validate a board
    for (int i = 0; i < 9; i++) {
      for (int j = 0; j < 9; j++) {
        char num = board[i][j];
        if (num != '.') {
          int n = board[i][j]-'1';
          int box_index = (i / 3 ) * 3 + j / 3;
          // keep the current cell value
          rows[i][n]++;
          columns[j][n]++;
          boxes[box_index][n]++;

          // check if this value has been already seen before
          if (rows[i][n] > 1 || columns[j][n] > 1 || boxes[box_index][n] > 1)
            return false;
        }
      }
    }

    return true;
  }
}