Valid Sudoku

Problem Statement:
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.

Intuition:
Main logic is to able to iterate over the matrix - key points

• have a map for every row, col, and box.
• for index of each box, the intution is
  1. "Each 3 rows, the box-row changes"
  2. "Each 3 cols, the box-col changes"
     that is boxi = (i // 3) * 3 + (j // 3)
     Code:

def solve(board):
	rows = defaultdict(set)
	cols = defaultdict(set)
	boxes = defaultdict(set)
	
	for i in range(9):
		for j in range(9):
			val = board[i][j]
			if val == '.':
				continue
			boxi = (i // 3) * 3 + (j // 3)
			if val in rows[i] or val in cols[j] or val in boxes[boxi]:
				return False
			
			rows[i].add(val)
			cols[j].add(val)
			boxes[boxi].add(val)
	
	return True

#array