LeetCode in Net

36. Valid Sudoku

Medium

Determine if a 9 x 9 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 nine 3 x 3 sub-boxes of the grid must contain the digits 1-9 without repetition.

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.

Example 1:

Input: board =

[[“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: board =

[[“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.

Constraints:

• board.length == 9
• board[i].length == 9
• board[i][j] is a digit 1-9 or '.'.

Solution

using System.Collections.Generic;

public class Solution {
    public bool IsValidSudoku(char[][] board) {
        bool[,] rows = new bool[9, 9];
        bool[,] cols = new bool[9, 9];
        bool[,] blocks = new bool[9, 9];
        for (int i = 0; i < board.Length; i++) {
            for (int j = i; j < board[i].Length; j++) {
                if (!isValidCase(board, i, j, rows, cols, blocks)) {
                    return false;
                }
                if (i != j && !isValidCase(board, j, i, rows, cols, blocks)) {
                    return false;
                }
            }
        }
        return true;
    }

    private bool isValidCase(char[][] board, int i, int j, bool[,] rows, bool[,] cols, bool[,] blocks) {
        if (board[i][j] == '.')
            return true;
        int num = board[i][j] - '1';
        int block = (i / 3) * 3 + (j / 3);
        if (rows[i, num] || cols[j, num] || blocks[block, num])
            return false;
        rows[i, num] = true;
        cols[j, num] = true;
        blocks[block, num] = true;
        return true;
    }
}

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