Andreas Hefti

I think there is a bug in

MaxRet(left.mostRetweeted,MaxRet(left.mostRetweeted,elem))

shouldn't it be?:

MaxRet(left.mostRetweeted,MaxRet(right.mostRetweeted,elem))

and instead of

if (left.isEmpty && right.isEmpty) elem
else if (right.isEmpty) MaxRet(left.mostRetweeted,elem)
else if (left.isEmpty) MaxRet(right.mostRetweeted,elem)
MaxRet(left.mostRetweeted,MaxRet(right.mostRetweeted,elem))

you can also write

MaxRet(
    if (left.isEmpty) elem else left.mostRetweeted,
    MaxRet(if (right.isEmpty) elem else right.mostRetweeted, elem)
)