input: Write a function to solve a given Sudoku puzzle represented as a list of lists. The function should return the solved board if a solution is possible; otherwise, it should return "No solution". If the provided board is empty, it should return an empty board.
  
  def solve_sudoku(board):
  
      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
  
      if board and board[0]:
          if (solve(board)):
              return board
          else:
              return "No solution"
      else:
          return []
  
  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]
  ]) == [
      [1, 2, 3, 4, 5, 6, 7, 8, 9],
      [4, 5, 6, 7, 8, 9, 1, 2, 3],
      [7, 8, 9, 1, 2, 3, 4, 5, 6],
      [2, 1, 4, 3, 6, 5, 8, 9, 7],
      [3, 6, 5, 8, 9, 7, 2, 1, 4],
      [8, 9, 7, 2, 1, 4, 3, 6, 5],
      [5, 3, 1, 6, 4, 2, 9, 7, 8],
      [6, 4, 2, 9, 7, 8, 5, 3, 1],
      [9, 7, 8, 5, 3, 1, 6, 4, 2]
  ]
  
  assert solve_sudoku([]) == []
  assert solve_sudoku([[]]) == []
output: def solve_sudoku(board):
      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
  
      if board and board[0]:
          if (solve(board)):
              return board
          else:
              return "No solution"
      else:
          return []