input: Write a function to solve a given Sudoku puzzle represented as a 2D list of integers. The function should fill in the empty cells (represented by 0) with appropriate values such that the entire board adheres to the Sudoku rules. If a solution exists, the function should return the solved board. If no solution exists, the function should indicate that there is no solution.
  
  
  board = [
      [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]
  ]
  
  assert solve_sudoku(board) == [
      [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]
  ]
  
  
  board_no_solution = [
      [5, 1, 6, 8, 4, 9, 7, 3, 2],
      [3, 0, 7, 6, 0, 5, 0, 0, 0],
      [8, 0, 9, 7, 0, 0, 0, 6, 5],
      [1, 3, 5, 0, 6, 0, 9, 0, 7],
      [4, 7, 2, 5, 9, 1, 0, 0, 6],
      [9, 6, 8, 3, 7, 0, 0, 5, 0],
      [2, 5, 3, 1, 8, 6, 0, 7, 4],
      [6, 8, 4, 2, 0, 7, 5, 0, 0],
      [7, 9, 1, 0, 5, 0, 6, 0, 8]
  ]
  
  assert solve_sudoku(board_no_solution) == 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 None