input: Write a function to post, get, and delete tweets for a given user.
  assert post_tweet('user1', 'Hello World!') is None
  assert get_tweets('user1') == ['Hello World!']
  assert delete_tweet('user1', 0) is None
  assert get_tweets('user1') == []
output: tweets_db = {}
  
  def post_tweet(user, tweet):
      if user not in tweets_db:
          tweets_db[user] = []
      tweets_db[user].append(tweet)
  
  def get_tweets(user):
      return tweets_db.get(user, [])
  
  def delete_tweet(user, tweet_index):
      if user in tweets_db and 0 <= tweet_index < len(tweets_db[user]):
          tweets_db[user].pop(tweet_index)