36. Valid Sudoku

Description of Problem

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

Explanation

Just hardcoded and use only one counter to reduce auxiliary space.

Code

impl Solution {
    pub fn is_valid_sudoku(board: Vec<Vec<char>>) -> bool {
        // assert!(board.len() == 9);
        // for i in 0..board.len() {
        //     assert!(board[i].len() == 9);
        // }

        let mut counter = vec![0; 9];

        for i in (0..9).step_by(3) {
            for j in (0..9).step_by(3) {
                Solution::count(&mut counter, board[i][j]);
                Solution::count(&mut counter, board[i][j+1]);
                Solution::count(&mut counter, board[i][j+2]);
                Solution::count(&mut counter, board[i+1][j]);
                Solution::count(&mut counter, board[i+1][j+1]);
                Solution::count(&mut counter, board[i+1][j+2]);
                Solution::count(&mut counter, board[i+2][j]);
                Solution::count(&mut counter, board[i+2][j+1]);
                Solution::count(&mut counter, board[i+2][j+2]);

                if counter.iter().any(|x| *x > 1) {
                    return false;
                }

                Solution::clear_count(&mut counter);

            }
        }

        for i in 0..9 {
            for j in 0..9 {
                Solution::count(&mut counter, board[i][j]);
            }
            if counter.iter().any(|x| *x > 1) {
                return false;
            }
            Solution::clear_count(&mut counter);
        }

        for j in 0..9 {
            for i in 0..9 {
                Solution::count(&mut counter, board[i][j]);
            }
            if counter.iter().any(|x| *x > 1) {
                return false;
            }
            Solution::clear_count(&mut counter);
        }

        return true;
    }

    fn count(counter : &mut [u32], ch: char) {
        match ch {
            '1' => counter[0]+=1,
            '2' => counter[1]+=1,
            '3' => counter[2]+=1,
            '4' => counter[3]+=1,
            '5' => counter[4]+=1,
            '6' => counter[5]+=1,
            '7' => counter[6]+=1,
            '8' => counter[7]+=1,
            '9' => counter[8]+=1,
            _   => {},
        }
    }

    fn clear_count(counter: &mut [u32]) {
        for i in 0..counter.len() {
            counter[i] = 0;
        }
    }
}