36. Valid Sudoku

Determine if a 9 x 9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules:

Each row must contain the digits 1-9 without repetition.
Each column must contain the digits 1-9 without repetition.
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 '.'.

 

class Solution:
    def isValidSudoku(self, board: List[List[str]]) -> bool:

        def check_row_and_col(r, c, n):
            for i in range(9):
                if (board[r][i] == n and i != c) or (board[i][c] == n and i != r):
                    return False

            # check rect
            start_row = (r // 3) * 3
            start_col = (c // 3) * 3
            for i in range(start_row, start_row + 3):
                for j in range(start_col, start_col + 3):
                    if board[i][j] == n and (i != r or j != c):
                        return False
            return True
        
        for i in range(9):
            for j in range(9):
                if board[i][j] != ".":
                    if not check_row_and_col(i, j, board[i][j]):
                        return False
        return True

 

스도쿠를 채우는 문제가 아니라

이 스도쿠가 유효한지, 무효한지만 검사하면 되는 문제

 

discussion을 보니

비교해야하는 인덱스 값이 현재 체크하는 인덱스값과 동일한지를 체크해야 하는 부분을

놓치는 부분이 많은 것 같다.

나도 놓쳐서 애먹었다 ㅋㅋ..