added assignment 4 ex 3

src/Assignment4/Ex3_Lists.sc

@@ -0,0 +1,64 @@
+import scala.annotation.tailrec
+
+
+// Complexity: O(n)
+@tailrec
+def last[T](list: List[T]): T = {
+  if (list.isEmpty) throw new NoSuchElementException()
+  if (list.tail.isEmpty) list.head
+  else last(list.tail)
+}
+
+def init[T](list: List[T]): List[T] = {
+  if (list.isEmpty) list
+  else if (list.tail.isEmpty) Nil
+  else list.head::init(list.tail)
+}
+
+// Complexity: O(n)
+def concat[T](l1: List[T], l2: List[T]): List[T] = {
+  if (l1.isEmpty) l2
+  else l1.head::concat(l1.tail, l2)
+}
+
+// Complexity: O(n²)
+def reverse[T](list: List[T]): List[T] = {
+  if (list.isEmpty) Nil
+  else last(list)::reverse(init(list))
+}
+
+// Better -> O(n)
+/*
+def reverse[T](list: List[T], res: List[T] = Nil): List[T] = {
+  if (list.isEmpty) res
+  else reverse(list.tail, list.head::res)
+}
+*/
+
+def take[T](list: List[T], n: Int): List[T] = {
+  if (list.isEmpty || n <= 0) Nil
+  else list.head::take(list.tail, n - 1)
+}
+
+@tailrec
+def drop[T](list: List[T], n: Int): List[T] = {
+  if (list.isEmpty || n <= 0) list
+  else drop(list.tail, n - 1)
+}
+
+@tailrec
+def apply[T](list: List[T], n: Int): T = {
+  if (list.isEmpty) throw new NoSuchElementException()
+  else if (n == 0) list.head
+  else apply(list.tail, n - 1)
+}
+
+
+assert(last(List(1,2,3)) == 3)
+assert(init(List(1,2,3)) == List(1,2))
+assert(concat(List(1,2,3), List(4,5,6)) == List(1,2,3,4,5,6))
+assert(reverse(List(1,2,3)) == List(3,2,1))
+assert(take(List(1,2,3), 2) == List(1,2))
+assert(drop(List(1,2,3), 2) == List(3))
+assert(drop(List(1,2,3), 4) == Nil)
+assert(apply(List(1,2,3), 2) == 3)