26.Answer

/*
We're using the method `max` that exists on all `Int` values rather than an explicit `if` expression.

Note how similar the implementation is to `size`. We'll abstract out the common pattern in a later exercise. 
*/
extension (t: Tree[Int]) def maximum: Int = t match
  case Leaf(n) => n
  case Branch(l, r) => l.maximum.max(r.maximum)