input: Write a python function to determine the winner of a rock-paper-scissors game.
  assert play_round("rock", "scissors") == "player"
  assert play_round("paper", "rock") == "player"
  assert play_round("scissors", "paper") == "player"
  assert play_round("rock", "rock") == "tie"
  assert play_round("scissors", "rock") == "ai"
  assert play_round("paper", "scissors") == "ai"
output: def play_round(player, ai):
      if player == ai:
          return "tie"
      elif (player == "rock" and ai == "scissors") or (player == "paper" and ai == "rock") or (player == "scissors" and ai == "paper"):
          return "player"
      else:
          return "ai"