Merge branch 'release/v1.0.22.0'

Author: andersonkmi

README.md

@@ -35,8 +35,11 @@ to keep up with the language and learn how to use the Scala test library as well
 * Contains duplicate
 * Middle of linked list
 * Jewels and stones
+* Reverse linked list
 
 ## Versions
+* v1.0.21.0 (10/12/2018) - Added Reverse linked list exercise.
+
 * v1.0.21.0 (10/07/2018) - Added Jewels and stones exercise.
 
 * v1.0.20.2 (10/06/2018) - Improved null argument handling for Middle of linked list exercise.

src/main/scala/org/codecraftlabs/leetcode/Main.scala

@@ -0,0 +1,26 @@
+package org.codecraftlabs.leetcode
+
+import scala.collection.mutable
+
+object ReverseLinkedList {
+  def reverseList(head: ListNode): ListNode = {
+    val nodeStack = mutable.ArrayStack[ListNode]()
+    process(head, nodeStack)
+    nodeStack.head
+  }
+
+  private def process(node: ListNode, nodeStack: mutable.ArrayStack[ListNode]): Unit = {
+    if (node != null) {
+      val item = node.next
+
+      val headItem = nodeStack.headOption
+      headItem match {
+        case None => node.next = null
+        case _ => node.next = headItem.get
+      }
+
+      nodeStack.push(node)
+      process(item, nodeStack)
+    }
+  }
+}

src/main/scala/org/codecraftlabs/leetcode/ReverseLinkedList.scala

@@ -0,0 +1,26 @@
+package org.codecraftlabs.leetcode
+
+import org.codecraftlabs.leetcode.ListNodeUtils.mkString
+import org.codecraftlabs.leetcode.ReverseLinkedList.reverseList
+import org.scalatest.{FlatSpec, Matchers}
+
+class ReverseLinkedListSpec extends FlatSpec with Matchers {
+  it should "return 3 -> 2 -> 1" in {
+    val node1 = new ListNode(1)
+    val node2 = new ListNode(2)
+    val node3 = new ListNode(3)
+
+    node1.next = node2
+    node2.next = node3
+
+    val result = mkString(reverseList(node1))
+    result shouldEqual "3-2-1"
+  }
+
+  it should "return 1" in {
+    val node1 = new ListNode(1)
+    val result = mkString(reverseList(node1))
+    result shouldEqual "1"
+  }
+}
+