Update 0036-valid-sudoku.py

Reduces unnecessary list indexing, making the code faster. Beats 99% on time on leetcode. consider merging

python/0036-valid-sudoku.py

@@ -6,16 +6,17 @@ def isValidSudoku(self, board: List[List[str]]) -> bool:
 
  for r in range(9):
  for c in range(9):
- if board[r][c] == ".":
+ num = board[r][c]
+ if num == ".":
  continue
  if (
- board[r][c] in rows[r]
- or board[r][c] in cols[c]
- or board[r][c] in squares[(r // 3, c // 3)]
+ num in rows[r]
+ or num in cols[c]
+ or num in squares[(r // 3, c // 3)]
  ):
  return False
- cols[c].add(board[r][c])
- rows[r].add(board[r][c])
- squares[(r // 3, c // 3)].add(board[r][c])
+ cols[c].add(num)
+ rows[r].add(num)
+ squares[(r // 3, c // 3)].add(num)
 
  return True