impl skip list

scala/src/main/scala/ch17_skip_list/SkipList.scala

@@ -0,0 +1,99 @@
+package ch17_skip_list
+
+import scala.util.Random
+
+class Node(var data: Int, var forwards: Array[Node], var maxLevel: Int)
+
+class SkipList(var head: Node, var skipListLevel: Int) {
+
+  def this() = this(new Node(-1, new Array[Node](16), 0), 1)
+
+  val MAX_LEVEL = 16
+  val random = new Random()
+
+  def find(value: Int): Option[Node] = {
+    var p = head
+    for (i <- skipListLevel - 1 to 0 by -1) {
+      while (p.forwards(i) != null && p.forwards(i).data < value) {
+        p = p.forwards(i)
+      }
+    }
+    if (p.forwards(0) != null && p.forwards(0).data == value) {
+      Some(p.forwards(0))
+    } else {
+      None
+    }
+  }
+
+  def insert(value: Int): Unit = {
+    //init the new node
+    val level = randomLevel()
+    val newNode = new Node(value, new Array[Node](level), level)
+
+    //use updtes array to record all nodes in all level before the inserted node
+    val updates: Array[Node] = new Array[Node](level)
+    var p = head
+    for (i <- level - 1 to 0 by -1) {
+      while (p.forwards(i) != null && p.forwards(i).data < value) {
+        p = p.forwards(i)
+      }
+      updates(i) = p
+    }
+
+    for (i <- Range(0, level)) {
+      newNode.forwards(i) = updates(i).forwards(i)
+      updates(i).forwards(i) = newNode
+    }
+
+    if (level > skipListLevel) {
+      skipListLevel = level
+    }
+  }
+
+  def delete(value: Int): Unit = {
+    var p = head
+    val updates: Array[Node] = new Array[Node](skipListLevel)
+
+    //try to locate the given node with the value
+    for (i <- skipListLevel - 1 to 0 by -1) {
+      while (p.forwards(i) != null && p.forwards(i).data < value) {
+        p = p.forwards(i)
+      }
+      updates(i) = p
+    }
+
+    if (p.forwards(0) != null && p.forwards(0).data == value) {
+      //find the value node, start to delete the node from the skip list
+      for (i <- skipListLevel - 1 to 0 by -1) {
+        if (updates(i).forwards(i) != null && updates(i).forwards(i).data == value) {
+          updates(i).forwards(i) = updates(i).forwards(i).forwards(i)
+        }
+      }
+    }
+
+  }
+
+  def randomLevel(): Int = {
+    var level = 1
+    for (i <- Range(1, MAX_LEVEL)) {
+      if (random.nextInt() % 2 == 1) {
+        level += 1
+      }
+    }
+
+    level
+  }
+
+  def mkString(): String = {
+    val builder = new StringBuilder
+    var p = head
+    while (p.forwards(0) != null) {
+      p = p.forwards(0)
+      builder.append(p.data)
+    }
+
+    builder.mkString
+  }
+}
+
+

scala/src/test/scala/ch17_skip_list/SkipListTest.scala

@@ -0,0 +1,44 @@
+package ch17_skip_list
+
+import org.scalatest.{FlatSpec, Matchers}
+
+import scala.util.Random
+
+class SkipListTest extends FlatSpec with Matchers {
+
+  behavior of "SkipListTest"
+
+  it should "insert skip list" in {
+    val list = new SkipList()
+    for (i <- Range(0, 10)) {
+      list.insert(i)
+    }
+
+    list.mkString() should equal("0123456789")
+  }
+
+  it should "delete skip list" in {
+    val list = new SkipList()
+    for (i <- Range(0, 10)) {
+      list.insert(i)
+    }
+
+    list.delete(5)
+    list.mkString() should equal("012346789")
+  }
+
+  it should "find value in skip list" in {
+    val list = new SkipList()
+    val length = 5000
+    val array = new Array[Int](length)
+    val rnd = new Random()
+    for (i <- Range(0, length)) {
+      array(i) = rnd.nextInt(length)
+      list.insert(array(i))
+    }
+
+    assert(list.find(array(rnd.nextInt(length - 1))).isDefined)
+    assert(list.find(array(rnd.nextInt(length - 1)) + length + 1).isEmpty)
+
+  }
+}