recursion

def max(list: List[Int]): Int = {
  if(list.isEmpty) Int.MinValue
  else list.head max max(list.tail)
}

def max2(list: List[Int]): Int = {
  if(list.tail.isEmpty) list.head
  else {
    val newMax = max(list.tail)
    if(list.head > newMax) list.head
    else newMax
  }
}

=====

Or we can use an accumulating parameter?  and something involving a helper function

def maxH(maxSoFar: Int, list: List[Int]): Int = {
  if (list.isEmpty) maxSoFar
  else maxH(maxSoFar max list.head, list.tail)
}