def mostRetweeted: Tweet = {
    if (left.isEmpty && right.isEmpty)
      elem
    else if (left.isEmpty)
      maxRetweeted(right.mostRetweeted, elem)
    else if (right.isEmpty)
      maxRetweeted(left.mostRetweeted, elem)
    else maxRetweeted(right.mostRetweeted, maxRetweeted(elem, left.mostRetweeted))
  }