DetectLoop done

Author: rampatra

src/main/java/com/ctci/linkedlists/LoopDetection.java

@@ -0,0 +1,65 @@
+package com.ctci.linkedlists;
+
+/**
+ * @author rampatra
+ * @since 2019-02-03
+ */
+public class LoopDetection {
+
+    /**
+     * Given a circular linked list, implement an algorithm that returns the node at the beginning of the loop.
+     * DEFINITION
+     * Circular linked list: A (corrupt) linked list in which a node's next pointer points to an earlier node, so
+     * as to make a loop in the linked list.
+     * EXAMPLE
+     * Input: A -> B -> C -> D -> E -> C [the same C as earlier]
+     * Output: C
+     *
+     * @param head the starting node of the linked list
+     * @return the {@code Node} where the loop starts, {@code null} otherwise.
+     */
+    private static Node findLoopStartNode(Node head) {
+        Node slow = head;
+        Node fast = head;
+
+        while (fast != null && fast.next != null) {
+            slow = slow.next;
+            fast = fast.next.next;
+            if (slow == fast) {
+                break;
+            }
+        }
+
+        /* Error check - no meeting point, and therefore no loop */
+        if (fast == null || fast.next == null) {
+            return null;
+        }
+
+        /* Move slow to Head. Keep fast at Meeting Point. Each are k steps from the
+         * Loop Start. If they move at the same pace, they must meet at Loop Start. */
+        slow = head;
+        while (slow != fast) {
+            slow = slow.next;
+            fast = fast.next;
+        }
+
+        /* You can return either as now both point to the start of the loop */
+        return fast;
+    }
+
+    public static void main(String[] args) {
+        /*
+         *   1 -> 2 -> 3 -> 4 -> 5 -> 6
+         *                  ^         |
+         *                  |_________|
+         */
+        Node l1 = new Node(1);
+        l1.next = new Node(2);
+        l1.next.next = new Node(3);
+        l1.next.next.next = new Node(4);
+        l1.next.next.next.next = new Node(5);
+        l1.next.next.next.next.next = new Node(6);
+        l1.next.next.next.next.next.next = l1.next.next.next;
+        System.out.println(findLoopStartNode(l1).val);
+    }
+}