impl deleteLastKthNode and test

scala/src/main/scala/ch07_linkedlist/LinkedListAlgo.scala

@@ -89,6 +89,40 @@ object LinkedListAlgo {
     head
   }
 
+  def deleteLastKthNode(headOpt: Option[Node], k: Int): Option[Node] = {
+    require(k > 0, "k must greater than 0")
+    headOpt match {
+      case None => None
+      case Some(head) =>
+        var index = 0
+        var slow = headOpt
+        var fast = headOpt
+        while (fast.get.next.isDefined && index < k) {
+          //move fast cursor to k
+          fast = fast.get.next
+          index += 1
+        }
+
+        if (fast.get.next.isEmpty && index + 1 == k) {
+          //deleting the head element
+          return head.next
+        }
+
+        require(index.equals(k), "given linked list should contains at least k elements ")
+        while (fast.get.next.isDefined) {
+          fast = fast.get.next
+          slow = slow.get.next
+        }
+
+        //fast cursor is in the end of the chain
+        //slow is the previous pos of k element
+        //do the operation
+        slow.get.next = slow.get.next.get.next
+    }
+
+    headOpt
+  }
+
   //form all the chain value as string
   def mkStringForChain(node: Node): String = {
     val result = new StringBuilder

scala/src/test/scala/ch07_linkedlist/LinkedListAlgoTest.scala

@@ -136,4 +136,73 @@ class LinkedListAlgoTest extends FlatSpec with Matchers {
     assert(node.isEmpty)
   }
 
+  it should "delete last 2nd element " in {
+    val list = new SinglyLinkedList()
+
+    for (i <- 0 to 9) {
+      list.insertTail(i)
+    }
+    LinkedListAlgo.mkStringForChain(list.headOpt.get) should equal((0 to 9).toArray.mkString(""))
+
+    val node = LinkedListAlgo.deleteLastKthNode(list.headOpt, 2)
+    assert(node.isDefined)
+    LinkedListAlgo.mkStringForChain(node.get) should equal("012345679")
+  }
+
+  it should "delete last element " in {
+    val list = new SinglyLinkedList()
+
+    for (i <- 0 to 9) {
+      list.insertTail(i)
+    }
+    LinkedListAlgo.mkStringForChain(list.headOpt.get) should equal((0 to 9).toArray.mkString(""))
+
+    val node = LinkedListAlgo.deleteLastKthNode(list.headOpt, 1)
+    assert(node.isDefined)
+    LinkedListAlgo.mkStringForChain(node.get) should equal("012345678")
+  }
+
+  it should "delete first element " in {
+    val list = new SinglyLinkedList()
+
+    for (i <- 0 to 9) {
+      list.insertTail(i)
+    }
+    LinkedListAlgo.mkStringForChain(list.headOpt.get) should equal((0 to 9).toArray.mkString(""))
+
+    val node = LinkedListAlgo.deleteLastKthNode(list.headOpt, 10)
+    assert(node.isDefined)
+    LinkedListAlgo.mkStringForChain(node.get) should equal("123456789")
+  }
+
+  it should "delete firs only element " in {
+    val list = new SinglyLinkedList()
+    list.insertTail(0)
+
+    val node = LinkedListAlgo.deleteLastKthNode(list.headOpt, 1)
+    assert(node.isEmpty)
+  }
+
+  it should "throw exception if k < 0 " in {
+    val list = new SinglyLinkedList()
+
+    for (i <- 0 to 9) {
+      list.insertTail(i)
+    }
+    assertThrows[IllegalArgumentException] {
+      LinkedListAlgo.deleteLastKthNode(list.headOpt, -1)
+    }
+  }
+
+  it should "throw exception if k greater than list length " in {
+    val list = new SinglyLinkedList()
+
+    for (i <- 0 to 9) {
+      list.insertTail(i)
+    }
+    assertThrows[IllegalArgumentException] {
+      LinkedListAlgo.deleteLastKthNode(list.headOpt, 15)
+    }
+  }
+
 }