input: def solve_sudoku(board):
      """
      Solve the Sudoku puzzle using backtracking.
      The board is a 9x9 grid represented as a list of lists.
      Each cell in the grid contains a number from 0 to 9, where 0 represents an empty cell.
      
      >>> 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]
      ... ]) is None
      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 or
                  board[i][col] == num or
                  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
      
      if solve(board):
          for row in board:
              print(row)
      else:
          print("No solution")
output:     solve_sudoku(board)