[LeetCode] 36. Valid Sudoku

Posted on | In

Problem

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.

36. Valid Sudoku

A partially filled sudoku which is valid.

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

Example 1:

1
2
3
4
5
6
7
8
9
10
11
12
13
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:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
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.

Explanation

  1. First we check every row if it contains duplicated number. We can use a boolean array of size 9 to mark if the number have been visited or not. If there are duplicated numbers, we can immediatly return false. Similarlly, after we check every row, we check every column. After that, we loop from 1 to 9 inclusive to check the 9 boxes.

Solution

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
class Solution {
    public boolean isValidSudoku(char[][] board) {
        for (int row = 0; row < 9; row++) {
            boolean[] rowToken = new boolean[9];
            for (int col = 0; col < 9; col++) {
                char charToken = board[row][col];
                if (charToken == '.') continue;
                int token = charToken - '0';
                if (rowToken[token-1] == true) {
                    // System.out.println("row");
                    return false;
                } else {
                    rowToken[token-1] = true;
                }
            }
        }
        
        for (int col = 0; col < 9; col++) {
            boolean[] colToken = new boolean[9];
            for (int row = 0; row < 9; row++) {
                char charToken = board[row][col];
                if (charToken == '.') continue;
                int token = charToken - '0';
                if (colToken[token-1] == true) {
                    // System.out.println("col");
                    return false;
                } else {
                    colToken[token-1] = true;
                }
            }
        }
        
        for (int box = 0; box < 9; box++) {
            boolean[] boxToken = new boolean[9];
            for (int row = 0; row < 3; row++) {
                for (int col = 0; col < 3; col++) {
                    char charToken = board[row + box/3*3][col + box%3*3];
                    if (charToken == '.') continue;
                    int token = charToken - '0';
                    if (boxToken[token-1] == true) {
                        // System.out.println("box");
                        return false;
                    } else {
                        boxToken[token-1] = true;
                    }
                }
            }
        }
        return true;
    }
}