input: 
  def solve_sudoku(board):
      """ Solves a given Sudoku puzzle.
      >>> solve_sudoku([
      ...     [7,8,0,4,0,0,1,2,0],
      ...     [6,0,0,0,7,5,0,0,9],
      ...     [0,0,0,6,0,1,0,7,8],
      ...     [0,0,7,0,4,0,2,6,0],
      ...     [0,0,1,0,5,0,9,3,0],
      ...     [9,0,4,0,6,0,0,0,5],
      ...     [0,7,0,3,0,0,0,1,2],
      ...     [1,2,0,0,0,7,4,0,0],
      ...     [0,4,9,2,0,6,0,0,7]
      ... ]) 
      True
      """
      def find_vacant_cell(board, l): 
          for row in range(9): 
              for col in range(9): 
                  if board[row][col] == 0: 
                      l[0] = row 
                      l[1] = col 
                      return True
          return False
    
      def is_safe(board, row, col, num): 
          for i in range(9): 
              if(board[row][i] == num): 
                  return False
              if(board[i][col] == num): 
                  return False
              if(board[(row//3)*3 + i//3][(col//3)*3 + i%3] == num): 
                  return False
          return True
    
      def solve(board): 
          l = [0, 0] 
          if (not find_vacant_cell(board, l)): 
              return True
          row = l[0] 
          col = l[1] 
          for num in range(1, 10): 
              if (is_safe(board, row, col, num)): 
                  board[row][col] = num 
                  if (solve(board)): 
                      return True
                  board[row][col] = 0
          return False
    
      return solve(board)
  
output:     return solve(board)