From 7d896490156b432b94f53a8b53f968a02f8f4bd3 Mon Sep 17 00:00:00 2001
From: Abhijit Sarkar <contact@asarkar.com>
Date: Thu, 4 Jan 2024 05:40:17 -0800
Subject: [PATCH] Complete up to P20

---
 README.md                          | 20 ++++++++++----------
 src/main/scala/list/P03.scala      |  2 +-
 src/main/scala/list/P07.scala      | 10 ++++++----
 src/main/scala/list/P10.scala      |  1 +
 src/main/scala/list/P11.scala      | 18 ++++++++++++++++++
 src/main/scala/list/P12.scala      | 14 ++++++++++++++
 src/main/scala/list/P13.scala      | 18 ++++++++++++++++++
 src/main/scala/list/P14.scala      | 11 +++++++++++
 src/main/scala/list/P15.scala      | 11 +++++++++++
 src/main/scala/list/P16.scala      | 16 ++++++++++++++++
 src/main/scala/list/P17.scala      | 16 ++++++++++++++++
 src/main/scala/list/P18.scala      | 16 ++++++++++++++++
 src/main/scala/list/P19.scala      | 16 ++++++++++++++++
 src/main/scala/list/P20.scala      | 16 ++++++++++++++++
 src/test/scala/list/P11Suite.scala | 21 +++++++++++++++++++++
 src/test/scala/list/P12Suite.scala | 15 +++++++++++++++
 src/test/scala/list/P13Suite.scala | 15 +++++++++++++++
 src/test/scala/list/P14Suite.scala | 14 ++++++++++++++
 src/test/scala/list/P15Suite.scala | 16 ++++++++++++++++
 src/test/scala/list/P16Suite.scala | 20 ++++++++++++++++++++
 src/test/scala/list/P17Suite.scala | 19 +++++++++++++++++++
 src/test/scala/list/P18Suite.scala | 20 ++++++++++++++++++++
 src/test/scala/list/P19Suite.scala | 15 +++++++++++++++
 src/test/scala/list/P20Suite.scala | 16 ++++++++++++++++
 24 files changed, 341 insertions(+), 15 deletions(-)
 create mode 100644 src/main/scala/list/P11.scala
 create mode 100644 src/main/scala/list/P12.scala
 create mode 100644 src/main/scala/list/P13.scala
 create mode 100644 src/main/scala/list/P14.scala
 create mode 100644 src/main/scala/list/P15.scala
 create mode 100644 src/main/scala/list/P16.scala
 create mode 100644 src/main/scala/list/P17.scala
 create mode 100644 src/main/scala/list/P18.scala
 create mode 100644 src/main/scala/list/P19.scala
 create mode 100644 src/main/scala/list/P20.scala
 create mode 100644 src/test/scala/list/P11Suite.scala
 create mode 100644 src/test/scala/list/P12Suite.scala
 create mode 100644 src/test/scala/list/P13Suite.scala
 create mode 100644 src/test/scala/list/P14Suite.scala
 create mode 100644 src/test/scala/list/P15Suite.scala
 create mode 100644 src/test/scala/list/P16Suite.scala
 create mode 100644 src/test/scala/list/P17Suite.scala
 create mode 100644 src/test/scala/list/P18Suite.scala
 create mode 100644 src/test/scala/list/P19Suite.scala
 create mode 100644 src/test/scala/list/P20Suite.scala

diff --git a/README.md b/README.md
index 4762e63..00c4f59 100644
--- a/README.md
+++ b/README.md
@@ -24,25 +24,25 @@
 
 [P10](src/main/scala/list/P10.scala) (*) Run-length encoding of a list.
 
-P11 (*) Modified run-length encoding.
+[P11](src/main/scala/list/P11.scala) (*) Modified run-length encoding.
 
-P12 (**) Decode a run-length encoded list.
+[P12](src/main/scala/list/P12.scala) (**) Decode a run-length encoded list.
 
-P13 (**) Run-length encoding of a list (direct solution).
+[P13](src/main/scala/list/P13.scala) (**) Run-length encoding of a list (direct solution).
 
-P14 (*) Duplicate the elements of a list.
+[P14](src/main/scala/list/P14.scala) (*) Duplicate the elements of a list.
 
-P15 (**) Duplicate the elements of a list a given number of times.
+[P15](src/main/scala/list/P15.scala) (**) Duplicate the elements of a list a given number of times.
 
-P16 (**) Drop every Nth element from a list.
+[P16](src/main/scala/list/P16.scala) (**) Drop every Nth element from a list.
 
-P17 (*) Split a list into two parts.
+[P17](src/main/scala/list/P17.scala) (*) Split a list into two parts.
 
-P18 (**) Extract a slice from a list.
+[P18](src/main/scala/list/P18.scala) (**) Extract a slice from a list.
 
-P19 (**) Rotate a list N places to the left.
+[P19](src/main/scala/list/P19.scala) (**) Rotate a list N places to the left.
 
-P20 (*) Remove the Kth element from a list.
+[P20](src/main/scala/list/P20.scala) (*) Remove the Kth element from a list.
 
 P21 (*) Insert an element at a given position into a list.
 
diff --git a/src/main/scala/list/P03.scala b/src/main/scala/list/P03.scala
index aaf7643..82e0b32 100644
--- a/src/main/scala/list/P03.scala
+++ b/src/main/scala/list/P03.scala
@@ -13,4 +13,4 @@ object P03:
   def nth[A](n: Int, l: List[A]): A = l match
     case x :: _ if n == 0 => x
     case x :: tail        => nth(n - 1, tail)
-    case Nil              => throw NoSuchElementException("n larger than list size")
+    case Nil              => throw NoSuchElementException("n too large")
diff --git a/src/main/scala/list/P07.scala b/src/main/scala/list/P07.scala
index 749a4c3..0ec1224 100644
--- a/src/main/scala/list/P07.scala
+++ b/src/main/scala/list/P07.scala
@@ -6,9 +6,11 @@ package list
 //     res0: List[Any] = List(1, 1, 2, 3, 5, 8)
 
 object P07:
-  def flatten(l: List[Any]): List[Any] = l.foldRight(Nil)(flatten)
+  def flatten[A](l: List[A | List[A]]): List[A] = l.foldRight(List.empty[A])(flatten)
 
-  private def flatten(x: Any, acc: List[Any]): List[Any] =
+  private def flatten[A](x: A | List[A], acc: List[A]): List[A] =
+    // Can't check for instanceOf(List[A]) since
+    // A vanishes at runtime (type erasure).
     if x.isInstanceOf[List[Any]]
-    then x.asInstanceOf[List[Any]].foldRight(acc)(flatten)
-    else x :: acc
+    then x.asInstanceOf[List[A]].foldRight(acc)(flatten)
+    else x.asInstanceOf[A] :: acc
diff --git a/src/main/scala/list/P10.scala b/src/main/scala/list/P10.scala
index 552da70..468c9de 100644
--- a/src/main/scala/list/P10.scala
+++ b/src/main/scala/list/P10.scala
@@ -13,4 +13,5 @@ package list
 object P10:
 
   def encode[A](l: List[A]): List[(Int, A)] =
+    // size takes O(n) time on lists.
     P09.pack(l).map(xs => (xs.size, xs.head))
diff --git a/src/main/scala/list/P11.scala b/src/main/scala/list/P11.scala
new file mode 100644
index 0000000..408cdb0
--- /dev/null
+++ b/src/main/scala/list/P11.scala
@@ -0,0 +1,18 @@
+package list
+
+// P11 (*) Modified run-length encoding.
+//     Modify the result of problem P10 in such a way that if an element has no
+//     duplicates it is simply copied into the result list.  Only elements with
+//     duplicates are transferred as (N, E) terms.
+//
+//     Example:
+//     scala> encodeModified(List('a, 'a, 'a, 'a, 'b, 'c, 'c, 'a, 'a, 'd, 'e, 'e, 'e, 'e))
+//     res0: List[Any] = List((4,'a), 'b, (2,'c), (2,'a), 'd, (4,'e))
+
+object P11:
+
+  def encodeModified[A](l: List[A]): List[A | (Int, A)] =
+    P09
+      .pack(l)
+      // size takes O(n) time on lists.
+      .map(xs => if xs.tail.isEmpty then xs.head else (xs.size, xs.head))
diff --git a/src/main/scala/list/P12.scala b/src/main/scala/list/P12.scala
new file mode 100644
index 0000000..a569609
--- /dev/null
+++ b/src/main/scala/list/P12.scala
@@ -0,0 +1,14 @@
+package list
+
+// P12 (**) Decode a run-length encoded list.
+//     Given a run-length code list generated as specified in problem P10,
+//     construct its uncompressed version.
+//
+//     Example:
+//     scala> decode(List((4, 'a), (1, 'b), (2, 'c), (2, 'a), (1, 'd), (4, 'e)))
+//     res0: List[Symbol] = List('a, 'a, 'a, 'a, 'b, 'c, 'c, 'a, 'a, 'd, 'e, 'e, 'e, 'e)
+
+object P12:
+
+  def decode[A](l: List[(Int, A)]): List[A] =
+    l.flatMap((len, x) => List.fill(len)(x))
diff --git a/src/main/scala/list/P13.scala b/src/main/scala/list/P13.scala
new file mode 100644
index 0000000..6c68cbe
--- /dev/null
+++ b/src/main/scala/list/P13.scala
@@ -0,0 +1,18 @@
+package list
+
+// P13 (**) Run-length encoding of a list (direct solution).
+//     Implement the so-called run-length encoding data compression method
+//     directly.  I.e. don't use other methods you've written (like P09's
+//     pack); do all the work directly.
+//
+//     Example:
+//     scala> encodeDirect(List('a, 'a, 'a, 'a, 'b, 'c, 'c, 'a, 'a, 'd, 'e, 'e, 'e, 'e))
+//     res0: List[(Int, Symbol)] = List((4,'a), (1,'b), (2,'c), (2,'a), (1,'d), (4,'e))
+
+object P13:
+
+  def encodeDirect[A](l: List[A]): List[(Int, A)] =
+    l.foldRight(List.empty[(Int, A)]):
+      case (x, Nil)                           => List((1, x))
+      case (x, acc @ ((_, y) :: _)) if x != y => (1, x) :: acc
+      case (_, (n, y) :: ys)                  => (n + 1, y) :: ys
diff --git a/src/main/scala/list/P14.scala b/src/main/scala/list/P14.scala
new file mode 100644
index 0000000..537d212
--- /dev/null
+++ b/src/main/scala/list/P14.scala
@@ -0,0 +1,11 @@
+package list
+
+// P14 (*) Duplicate the elements of a list.
+//     Example:
+//     scala> duplicate(List('a, 'b, 'c, 'c, 'd))
+//     res0: List[Symbol] = List('a, 'a, 'b, 'b, 'c, 'c, 'c, 'c, 'd, 'd)
+
+object P14:
+
+  def duplicate[A](l: List[A]): List[A] =
+    l.flatMap(x => List(x, x))
diff --git a/src/main/scala/list/P15.scala b/src/main/scala/list/P15.scala
new file mode 100644
index 0000000..36dcd10
--- /dev/null
+++ b/src/main/scala/list/P15.scala
@@ -0,0 +1,11 @@
+package list
+
+// P15 (**) Duplicate the elements of a list a given number of times.
+//     Example:
+//     scala> duplicateN(3, List('a, 'b, 'c, 'c, 'd))
+//     res0: List[Symbol] = List('a, 'a, 'a, 'b, 'b, 'b, 'c, 'c, 'c, 'c, 'c, 'c, 'd, 'd, 'd)
+
+object P15:
+
+  def duplicateN[A](n: Int, l: List[A]): List[A] =
+    l.flatMap(x => List.fill(n)(x))
diff --git a/src/main/scala/list/P16.scala b/src/main/scala/list/P16.scala
new file mode 100644
index 0000000..ffdb5a8
--- /dev/null
+++ b/src/main/scala/list/P16.scala
@@ -0,0 +1,16 @@
+package list
+
+// P16 (**) Drop every Nth element from a list.
+//     Example:
+//     scala> drop(3, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
+//     res0: List[Symbol] = List('a, 'b, 'd, 'e, 'g, 'h, 'j, 'k)
+
+object P16:
+
+  def drop[A](n: Int, l: List[A]): List[A] =
+    // Other ways:
+    //   xs.withFilter(...).map(...)
+    //   for (..) yield
+    //   xs.flatMap(...)
+    l.zipWithIndex.collect:
+      case (x, i) if (i + 1) % n != 0 => x
diff --git a/src/main/scala/list/P17.scala b/src/main/scala/list/P17.scala
new file mode 100644
index 0000000..2b2f667
--- /dev/null
+++ b/src/main/scala/list/P17.scala
@@ -0,0 +1,16 @@
+package list
+
+// P17 (*) Split a list into two parts.
+//     The length of the first part is given.  Use a Tuple for your result.
+//
+//     Example:
+//     scala> split(3, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
+//     res0: (List[Symbol], List[Symbol]) = (List('a, 'b, 'c),List('d, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
+object P17:
+
+  def split[A](n: Int, l: List[A]): (List[A], List[A]) = l match
+    case Nil          => (Nil, Nil)
+    case xs if n == 0 => (Nil, xs)
+    case x :: xs =>
+      split((n - 1), xs) match
+        case (left, right) => (x :: left, right)
diff --git a/src/main/scala/list/P18.scala b/src/main/scala/list/P18.scala
new file mode 100644
index 0000000..9ad6ac6
--- /dev/null
+++ b/src/main/scala/list/P18.scala
@@ -0,0 +1,16 @@
+package list
+
+// P18 (**) Extract a slice from a list.
+//     Given two indices, I and K, the slice is the list containing the elements
+//     from and including the Ith element up to but not including the Kth
+//     element of the original list.  Start counting the elements with 0.
+//
+//     Example:
+//     scala> slice(3, 7, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
+//     res0: List[Symbol] = List('d, 'e, 'f, 'g)
+object P18:
+
+  def slice[A](start: Int, end: Int, l: List[A]): List[A] =
+    if end < start
+    then throw new IllegalArgumentException("invalid bounds")
+    else P17.split(start, l)(1).take(end - start)
diff --git a/src/main/scala/list/P19.scala b/src/main/scala/list/P19.scala
new file mode 100644
index 0000000..bf0e515
--- /dev/null
+++ b/src/main/scala/list/P19.scala
@@ -0,0 +1,16 @@
+package list
+
+// P19 (**) Rotate a list N places to the left.
+//     Examples:
+//     scala> rotate(3, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
+//     res0: List[Symbol] = List('d, 'e, 'f, 'g, 'h, 'i, 'j, 'k, 'a, 'b, 'c)
+//
+//     scala> rotate(-2, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
+//     res1: List[Symbol] = List('j, 'k, 'a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i)
+
+object P19:
+
+  def rotate[A](n: Int, l: List[A]): List[A] =
+    val k             = Math.floorMod(n, l.size)
+    val (left, right) = P17.split(k, l)
+    right ++ left
diff --git a/src/main/scala/list/P20.scala b/src/main/scala/list/P20.scala
new file mode 100644
index 0000000..39bef31
--- /dev/null
+++ b/src/main/scala/list/P20.scala
@@ -0,0 +1,16 @@
+package list
+
+// P20 (*) Remove the Kth element from a list.
+//     Return the list and the removed element in a Tuple.  Elements are
+//     numbered from 0.
+//
+//     Example:
+//     scala> removeAt(1, List('a, 'b, 'c, 'd))
+//     res0: (List[Symbol], Symbol) = (List('a, 'c, 'd),'b)
+
+object P20:
+
+  def removeAt[A](n: Int, l: List[A]): (List[A], A) =
+    P17.split(n, l) match
+      case (_, Nil)      => throw new NoSuchElementException("n too large")
+      case (left, right) => (left ++ right.tail, right.head)
diff --git a/src/test/scala/list/P11Suite.scala b/src/test/scala/list/P11Suite.scala
new file mode 100644
index 0000000..2793820
--- /dev/null
+++ b/src/test/scala/list/P11Suite.scala
@@ -0,0 +1,21 @@
+package list
+
+import munit.ScalaCheckSuite
+import org.scalacheck.Prop.forAllNoShrink
+import org.scalacheck.Gen
+
+class P11Suite extends ScalaCheckSuite:
+
+  property("modified run-length encoding"):
+    forAllNoShrink(Gen.listOf(Gen.choose(1, 5))) { (lst: List[Int]) =>
+      val xs       = lst.flatMap(n => List.fill(n)(n))
+      val obtained = P11.encodeModified(xs)
+      val expected = xs
+        .foldRight(List.empty[(Int, Int)])((x, acc) =>
+          if (acc.isEmpty || acc.head(1) != x)
+          then (1, x) :: acc
+          else (acc.head(0) + 1, acc.head(1)) :: acc.tail
+        )
+        .map((len, x) => if len == 1 then x else (len, x))
+      assertEquals(obtained, expected)
+    }
diff --git a/src/test/scala/list/P12Suite.scala b/src/test/scala/list/P12Suite.scala
new file mode 100644
index 0000000..ff29e96
--- /dev/null
+++ b/src/test/scala/list/P12Suite.scala
@@ -0,0 +1,15 @@
+package list
+
+import munit.ScalaCheckSuite
+import org.scalacheck.Prop.forAllNoShrink
+import org.scalacheck.Gen
+
+class P12Suite extends ScalaCheckSuite:
+
+  property("decode run-length encoded list"):
+    forAllNoShrink(Gen.listOf(Gen.choose(1, 5))) { (lst: List[Int]) =>
+      val xs       = lst.map(x => (x, x))
+      val obtained = P12.decode(xs)
+      val expected = lst.flatMap(x => List.fill(x)(x))
+      assertEquals(obtained, expected)
+    }
diff --git a/src/test/scala/list/P13Suite.scala b/src/test/scala/list/P13Suite.scala
new file mode 100644
index 0000000..1a6f220
--- /dev/null
+++ b/src/test/scala/list/P13Suite.scala
@@ -0,0 +1,15 @@
+package list
+
+import munit.ScalaCheckSuite
+import org.scalacheck.Prop.forAllNoShrink
+import org.scalacheck.Gen
+
+class P13Suite extends ScalaCheckSuite:
+
+  property("run-length encoding direct"):
+    forAllNoShrink(Gen.listOf(Gen.choose(1, 5))) { (lst: List[Int]) =>
+      val xs       = lst.flatMap(n => List.fill(n)(n))
+      val obtained = P13.encodeDirect(xs)
+      val expected = P10.encode(xs)
+      assertEquals(obtained, expected)
+    }
diff --git a/src/test/scala/list/P14Suite.scala b/src/test/scala/list/P14Suite.scala
new file mode 100644
index 0000000..f91a2e2
--- /dev/null
+++ b/src/test/scala/list/P14Suite.scala
@@ -0,0 +1,14 @@
+package list
+
+import munit.ScalaCheckSuite
+import org.scalacheck.Prop.forAllNoShrink
+import org.scalacheck.Gen
+
+class P14Suite extends ScalaCheckSuite:
+
+  property("duplicate the elements"):
+    forAllNoShrink(Gen.listOf(ListGen.genInt)) { (lst: List[Int]) =>
+      val obtained = P14.duplicate(lst)
+      val expected = lst.flatMap(n => List.fill(2)(n))
+      assertEquals(obtained, expected)
+    }
diff --git a/src/test/scala/list/P15Suite.scala b/src/test/scala/list/P15Suite.scala
new file mode 100644
index 0000000..3e6778b
--- /dev/null
+++ b/src/test/scala/list/P15Suite.scala
@@ -0,0 +1,16 @@
+package list
+
+import munit.ScalaCheckSuite
+import org.scalacheck.Prop.forAllNoShrink
+import org.scalacheck.Gen
+import scala.util.Random
+
+class P15Suite extends ScalaCheckSuite:
+
+  property("duplicate the elements N times"):
+    forAllNoShrink(Gen.listOf(ListGen.genInt)) { (lst: List[Int]) =>
+      val n        = Random.between(1, 5)
+      val obtained = P15.duplicateN(n, lst)
+      val expected = lst.flatMap(x => List.fill(n)(x))
+      assertEquals(obtained, expected)
+    }
diff --git a/src/test/scala/list/P16Suite.scala b/src/test/scala/list/P16Suite.scala
new file mode 100644
index 0000000..6f4b1c6
--- /dev/null
+++ b/src/test/scala/list/P16Suite.scala
@@ -0,0 +1,20 @@
+package list
+
+import munit.ScalaCheckSuite
+import org.scalacheck.Prop.forAllNoShrink
+import org.scalacheck.Gen
+import scala.util.Random
+
+class P16Suite extends ScalaCheckSuite:
+
+  property("drop every Nth element"):
+    forAllNoShrink(Gen.listOf(ListGen.genInt)) { (lst: List[Int]) =>
+      val n =
+        if lst.isEmpty
+        then 0
+        else Random.between(1, lst.size + 1)
+      val obtained = P16.drop(n, lst)
+      // guard in for-comprehension is sugar for withFilter.
+      val expected = for ((x, i) <- lst.zipWithIndex if (i + 1) % n != 0) yield x
+      assertEquals(obtained, expected)
+    }
diff --git a/src/test/scala/list/P17Suite.scala b/src/test/scala/list/P17Suite.scala
new file mode 100644
index 0000000..2b3e202
--- /dev/null
+++ b/src/test/scala/list/P17Suite.scala
@@ -0,0 +1,19 @@
+package list
+
+import munit.ScalaCheckSuite
+import org.scalacheck.Prop.forAllNoShrink
+import org.scalacheck.Gen
+import scala.util.Random
+
+class P17Suite extends ScalaCheckSuite:
+
+  property("split a list into two parts"):
+    forAllNoShrink(Gen.listOf(ListGen.genInt)) { (lst: List[Int]) =>
+      val n =
+        if lst.isEmpty
+        then 0
+        else Random.between(1, lst.size + 1)
+      val obtained = P17.split(n, lst)
+      val expected = lst.splitAt(n)
+      assertEquals(obtained, expected)
+    }
diff --git a/src/test/scala/list/P18Suite.scala b/src/test/scala/list/P18Suite.scala
new file mode 100644
index 0000000..8efd075
--- /dev/null
+++ b/src/test/scala/list/P18Suite.scala
@@ -0,0 +1,20 @@
+package list
+
+import munit.ScalaCheckSuite
+import org.scalacheck.Prop.forAllNoShrink
+import org.scalacheck.Gen
+import scala.util.Random
+
+class P18Suite extends ScalaCheckSuite:
+
+  property("extract a slice"):
+    forAllNoShrink(Gen.listOf(ListGen.genInt)) { (lst: List[Int]) =>
+      val end =
+        if lst.isEmpty
+        then 0
+        else Random.between(1, lst.size + 1)
+      val start    = if end == 0 then 0 else Random.between(0, end)
+      val obtained = P18.slice(start, end, lst)
+      val expected = lst.splitAt(start)._2.take(end - start)
+      assertEquals(obtained, expected, s"start=${start}, end=${end}")
+    }
diff --git a/src/test/scala/list/P19Suite.scala b/src/test/scala/list/P19Suite.scala
new file mode 100644
index 0000000..5349ca2
--- /dev/null
+++ b/src/test/scala/list/P19Suite.scala
@@ -0,0 +1,15 @@
+package list
+
+import munit.FunSuite
+
+class P19Suite extends FunSuite:
+
+  test("rotate a list N places to the left"):
+    val xs        = ('a' to 'k').toList
+    val obtained1 = P19.rotate(3, xs)
+    val expected1 = List('d', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'a', 'b', 'c')
+    assertEquals(obtained1, expected1)
+
+    val obtained2 = P19.rotate(-2, xs)
+    val expected2 = List('j', 'k', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i')
+    assertEquals(obtained2, expected2)
diff --git a/src/test/scala/list/P20Suite.scala b/src/test/scala/list/P20Suite.scala
new file mode 100644
index 0000000..f519501
--- /dev/null
+++ b/src/test/scala/list/P20Suite.scala
@@ -0,0 +1,16 @@
+package list
+
+import munit.ScalaCheckSuite
+import org.scalacheck.Prop.forAllNoShrink
+import scala.util.Random
+
+class P20Suite extends ScalaCheckSuite:
+
+  property("remove the Kth element"):
+    forAllNoShrink(ListGen.genNotEmpty) { (lst: List[Int]) =>
+      val k             = Random.between(0, lst.size)
+      val obtained      = P20.removeAt(k, lst)
+      val (left, right) = lst.splitAt(k)
+      val expected      = (left ++ right.tail, right.head)
+      assertEquals(obtained, expected, s"k=${k}")
+    }