input: Write a function to solve a Sudoku puzzle given an initial board configuration. If a solution exists, return the solved board; otherwise, print "No solution". If the given board is empty then return an empty list.
  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]
  ])=="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],
      [1,2,0,0,0,0,4,0,0],
      [0,4,9,2,0,6,0,0,7]
  ])==[
      [5, 3, 4, 6, 7, 8, 9, 1, 2],
      [6, 7, 2, 1, 9, 5, 3, 4, 8],
      [1, 9, 8, 3, 4, 2, 5, 6, 7],
      [8, 5, 9, 7, 6, 1, 4, 2, 3],
      [4, 2, 6, 8, 5, 3, 7, 9, 1],
      [7, 1, 3, 9, 2, 4, 8, 5, 6],
      [9, 6, 1, 5, 3, 7, 2, 8, 4],
      [2, 8, 7, 4, 1, 9, 6, 3, 5],
      [3, 4, 5, 2, 8, 6, 1, 7, 9]
  ]
  assert solve_sudoku([])==[]
output: def solve_sudoku(board):
      if not board:
          return []
      
      def is_valid(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):
          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"