issue-134: find loop linkedlist node

src/main/java/com/ordestiny/tdd/interview/linkedlist/LinkedListNodeUtil.java

@@ -61,4 +61,45 @@ public static LinkedListNode partition(LinkedListNode node , int x) {
 
  return leftNodeHead;
  }
+
+ /**
+ * interview Question:
+ * Detect a loop linkedlist node
+ * Given a list node
+ * When node link as (A->B->C-D->B)
+ * Expect return B
+ * When node link as (C->D->A->B->E->D)
+ * Expect return C
+ *
+ * @param head
+ * @return
+ */
+ public static LinkedListNode findLoopNode(LinkedListNode head) {
+ LinkedListNode slowNode = head;
+ LinkedListNode fastNode = head;
+
+ // find first meet node,
+ // k:(distance from list start to loop node start)
+ //
+ // number of nodes passed should be k + (circle length - mod(k, circle length))
+ while (fastNode != null && fastNode.hasNext()) {
+ slowNode = slowNode.next;
+ fastNode = fastNode.next.next;
+ if (slowNode == fastNode)
+ break;
+ }
+
+ // no meet
+ if (fastNode == null || fastNode.next == null)
+ return null;
+
+
+ slowNode = head;
+ while(slowNode != fastNode) {
+ slowNode = slowNode.next;
+ fastNode = fastNode.next;
+ }
+
+ return fastNode;
+ }
 }

src/test/java/com/ordestiny/tdd/interview/linkedlist/LinkedListNodeUtilTest.java

@@ -54,4 +54,29 @@ private static LinkedListNode generate(int... nodeValues) {
  return node;
  }
 
+ @ParameterizedTest
+ @MethodSource("findLoopNodeArguments")
+ void foundLoopNode(LinkedListNode<Integer> node, int loopNodeIndex) {
+ LinkedListNode loopNode = node;
+ for (int i=0;i<loopNodeIndex;i++) {
+ loopNode = loopNode.next;
+ }
+ LinkedListNode endNode = loopNode;
+ while(endNode.hasNext())
+ endNode = endNode.next;
+
+ endNode.next = loopNode;
+ LinkedListNode returnNode = util.findLoopNode(node);
+ Assertions.assertEquals(loopNode.getData(), returnNode.getData());
+ }
+
+ private static Stream<Arguments> findLoopNodeArguments() {
+ return Stream.of(
+ Arguments.of(generate(3, 5, 8, 4, 10, 2, 1), 5),
+ Arguments.of(generate(3, 11, 20, 7, 9, 10), 3),
+ Arguments.of(generate(20, 19, 23, 13, 6, 5, 0), 5),
+ Arguments.of(generate(20, 19, 23, 13, 6, 5, 0), 4),
+ Arguments.of(generate(20), 0)
+ );
+ }
 }