detect loop in link list java

Author: prtk418

linked_list/detect_loop_in_link_list.java

@@ -0,0 +1,74 @@
+// Java program to detect loop in a linked list
+import java.util.*;
+ 
+public class LinkedList {
+     
+    static Node head;  // head of list
+       
+    /* Linked list Node*/
+    static class Node
+    {
+        int data;
+        Node next;
+        Node(int d) {data = d; next = null; }
+    }
+   
+    /* Inserts a new Node at front of the list. */
+    static public void push(int new_data)
+    {
+        /* 1 & 2: Allocate the Node &
+                  Put in the data*/
+        Node new_node = new Node(new_data);
+   
+        /* 3. Make next of new Node as head */
+        new_node.next = head;
+   
+        /* 4. Move the head to point to new Node */
+        head = new_node;
+    }
+  
+    // Returns true if there is a loop in linked
+    // list else returns false.
+    static boolean detectLoop(Node h)
+    {
+        HashSet<Node> s = new HashSet<Node>();
+        while (h != null)
+        {
+             // If we have already has this node
+             // in hashmap it means their is a cycle
+             // (Because you we encountering the
+             // node second time).
+            if (s.contains(h))
+                return true;
+   
+            // If we are seeing the node for
+            // the first time, insert it in hash
+            s.add(h);
+   
+            h = h.next;
+        }
+   
+        return false;
+    }
+     
+    /* Driver program to test above function */
+    public static void main(String[] args) 
+    {
+        LinkedList llist = new LinkedList();
+          
+        llist.push(46);
+        llist.push(78);
+        llist.push(52);
+        llist.push(94);
+          
+        /*Create loop for testing */
+        llist.head.next.next.next.next = llist.head;
+  
+        if (detectLoop(head))
+            System.out.println("Loop found");
+        else
+            System.out.println("No Loop");
+     
+    }
+}
+