dev calculateCircleLength and findCircleEntrance

Author: ivan

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

@@ -2,6 +2,8 @@ package ch07_linkedlist
 
 import ch06_linkedlist.Node
 
+import scala.util.control.Breaks._
+
 object LinkedListAlgo {
 
   //reverse a linked list
@@ -27,7 +29,12 @@ object LinkedListAlgo {
     current.get
   }
 
-  def checkCircle(head: Node): Boolean = {
+  /**
+    *
+    * @param head
+    * @return Some(Node) a node in a circle or None
+    */
+  def checkCircle(head: Node): Option[Node] = {
     var fast = head
     var slow = head
 
@@ -36,10 +43,47 @@ object LinkedListAlgo {
       slow = slow.next.get
 
       if (fast.equals(slow)) {
-        return true
+        return Some(slow)
       }
     }
-    false
+    None
+  }
+
+  /**
+    * calculate the length of the circle
+    *
+    * @param node - some node in the circle
+    * @return circle length
+    */
+  def calculateCircleLength(node: Node): Int = {
+    var length = 1
+    var cursor = node.next.get
+
+    while (cursor != node) {
+      length += 1
+      cursor = cursor.next.get
+    }
+
+    length
+  }
+
+  def findCircleEntrance(head: Node): Option[Node] = {
+    checkCircle(head).map(node => {
+      val length = calculateCircleLength(node)
+      var fast = head
+      var slow = head
+      //fast move length steps
+      for (i <- 0 until length) {
+        fast = fast.next.get
+      }
+
+      while (slow != fast) {
+        fast = fast.next.get
+        slow = slow.next.get
+      }
+
+      slow
+    })
   }
 
   //assuming nodeA and nodeB are all sorted list in ascending order

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

@@ -50,7 +50,7 @@ class LinkedListAlgoTest extends FlatSpec with Matchers {
     node5.next = Some(node6)
     node6.next = Some(node3)
 
-    assert(LinkedListAlgo.checkCircle(node1))
+    assert(LinkedListAlgo.checkCircle(node1).isDefined)
   }
 
   it should "check circle for none circled linked list" in {
@@ -70,7 +70,53 @@ class LinkedListAlgoTest extends FlatSpec with Matchers {
     node4.next = Some(node5)
     node5.next = Some(node6)
 
-    assert(!LinkedListAlgo.checkCircle(node1))
+    assert(LinkedListAlgo.checkCircle(node1).isEmpty)
+  }
+
+  it should "calculate circle length" in {
+    val node1 = new Node(1, None)
+    val node2 = new Node(2, None)
+    val node3 = new Node(3, None)
+    val node4 = new Node(4, None)
+    val node5 = new Node(5, None)
+    val node6 = new Node(6, None)
+
+    //1->2->3->4->5->6->3 it's a circle
+    //node1 is the head
+    node1.next = Some(node2)
+    node2.next = Some(node3)
+    node3.next = Some(node4)
+    node4.next = Some(node5)
+    node5.next = Some(node6)
+    node6.next = Some(node3)
+
+    val node = LinkedListAlgo.checkCircle(node1).get
+
+    val length = LinkedListAlgo.calculateCircleLength(node)
+
+    length should equal(4)
+  }
+
+  it should "find entrance of the  circle" in {
+    val node1 = new Node(1, None)
+    val node2 = new Node(2, None)
+    val node3 = new Node(3, None)
+    val node4 = new Node(4, None)
+    val node5 = new Node(5, None)
+    val node6 = new Node(6, None)
+
+    //1->2->3->4->5->6->3 it's a circle
+    //node1 is the head
+    node1.next = Some(node2)
+    node2.next = Some(node3)
+    node3.next = Some(node4)
+    node4.next = Some(node5)
+    node5.next = Some(node6)
+    node6.next = Some(node3)
+
+    val node = LinkedListAlgo.findCircleEntrance(node1)
+
+    node.get.data should equal(3)
   }
 
   it should "merge 2 sorted list into 1" in {