【Leetcode】36. Valid Sudoku

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: true

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: false


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.

Note:

• A Sudoku board (partially filled) could be valid but is not necessarily solvable.
• Only the filled cells need to be validated according to the mentioned rules.
• The given board contain only digits 1-9 and the character ‘.’.
• The given board size is always 9x9.

Discuss

判断是否是一个数独，只要满足数独的条件即可。使用的是最直观的判断方法，还有使用多个boolean数组来判断的简便方法。

Code

public boolean isValidSudoku(char[][] board) {
        int[] a= new int[10];
        for (int i = 0; i < 9; i++) {
            a = new int[10];
            for (int j = 0; j < 9; j++) {
                if (board[i][j] == '.') { continue; }
                int c = board[i][j] - '0';
                if (c > 9 || c < 1) { return false; }
                if(a[c] != 0) { return false; }
                a[c] = 1;
            }
        }
        for (int i = 0; i < 9; i++) {
            a = new int[10];
            for (int j = 0; j < 9; j++) {
                if (board[j][i] == '.') { continue; }
                int c = board[j][i] - '0';
                if (c > 9 || c < 1) { return false; }
                if (a[c] != 0) { return false; }
                a[c] = 1;
            }
        }
        for (int i = 0; i < 3; i++) {
            for (int j=  0; j < 3; j++) {
                a = new int[10];
                for (int k = 3 * i; k < 3 * (i + 1); k++) {
                    for (int l = 3 * j; l < 3 * (j + 1); l++) {
                        if (board[k][l] == '.') { continue; }
                        int c = board[k][l] - '0';
                        if (c < 1 || c > 9) { return false; }
                        if (a[c] != 0) { return false; }
                        a[c] = 1;
                    }
                }
            }
        }
        return true;
    }