input: Write a function to solve a given Sudoku puzzle. The function accepts a 9x9 Sudoku board as input and modifies it to be filled with the solution if possible. If the board is empty return "No solution".
  assert 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]
  ])==[
      [7,8,5,4,3,9,1,2,6],
      [6,1,2,8,7,5,3,4,9],
      [4,9,3,6,2,1,5,7,8],
      [8,5,7,9,4,3,2,6,1],
      [2,6,1,7,5,8,9,3,4],
      [9,3,4,1,6,2,7,8,5],
      [5,7,8,3,9,4,6,1,2],
      [1,2,6,5,8,7,4,9,3],
      [3,4,9,2,1,6,8,5,7]
  ]
  assert solve_sudoku([
      [0,0,0,0,0,0,0,0,0],
      [0,0,0,0,0,0,0,0,0],
      [0,0,0,0,0,0,0,0,0],
      [0,0,0,0,0,0,0,0,0],
      [0,0,0,0,0,0,0,0,0],
      [0,0,0,0,0,0,0,0,0],
      [0,0,0,0,0,0,0,0,0],
      [0,0,0,0,0,0,0,0,0],
      [0,0,0,0,0,0,0,0,0]
  ])==None
output: def solve_sudoku(board):
      def is_valid(board, row, col, num):
          for i in range(9):
              if board[row][i] == num or board[i][col] == num or board[3 * (row // 3) + i // 3][3 * (col // 3) + i % 3] == num:
                  return False
          return True
  
      def solve(board):
          for row in range(9):
              for col in range(9):
                  if board[row][col] == 0:
                      for num in range(1, 10):
                          if is_valid(board, row, col, num):
                              board[row][col] = num
                              if solve(board):
                                  return True
                              board[row][col] = 0
                      return False
          return True
  
      if solve(board):
          return board
      else:
          return "No solution"