#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:
   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.

 
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.

from collections import defaultdict

class Solution:

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

        n = len(board)

        rows = defaultdict(list)

        cols = defaultdict(list)

        box = defaultdict(list)

        for r in range(n):

            for c in range(n):

                value = board[r][c]

                if value == ".":

                    continue

                # 1. Rows Check      

                if value in rows[r]:

                    return False

                rows[r].append(value)

                # 2. Cols Check

                if value in cols[c]:

                    return False

                cols[c].append(value)

                # 3. calculate each box

                index = (r//3) * 3 + c//3  

                if value in box[index]:

                    return False

                box[index].append(value)

        return True

               

https://leetcode.com/problems/valid-sudoku/description/?envType=featured-list&envId=top-interview-questions?envType=featured-list&envId=top-interview-questions