Link Lists(Nuts & Bolts of CS)

Below diagram shows a linked list with a loop. detectAndRemoveLoop() must change the below list to 1->2->3->4->5->NULL.We can also use Floyd Cycle Detection algorithm to detect and remove the loop. In the Floyd’s algo, the slow and fast pointers meet at a loop node. We can use this loop node to remove cycle. There are following two different ways of removing loop when Floyd’s algorithm is used for Loop detection.

CODE-

class Node {
 Object data;
 Node next;
 }

public class FloydCycleFindingAlgorithm {

/**
 * Detects and removes a cycle in a linked list. Returns true if a cycle was
 * found and removed; or false otherwise.
 */
 public static boolean removeCycle( Node head ) {

Node tortoise = head;
 Node hare = head.next;

// Search for a cycle
 while( tortoise != hare && hare.next != null )
 { // 2 condition: if not null then must be loop. this cond. can be removed also
 tortoise = tortoise.next;
 System.out.println( "tortoise is = "+tortoise.data );

hare = hare.next.next; //hare.next=B,b.next=C
 System.out.println( "hare is = "+hare.data );

}

// Check if a cycle was found
 if( tortoise != hare ) {
 return false;
 }

else
 {
 // Find the node from which the cycle starts
 Node tort = head; /// tort =B, hare=D
 while( hare.next != tort ) // D.next!=B ; increment both
 {
 tort = tort.next;
 hare = hare.next;
 }

// Remove cycle
 hare.next = null;
 }
 return true;
 }

/**
 * Test Method
 */
 public static void main( String[ ] args ) {

// Initialize the linked-list nodes
 //
 Node nodeA = new Node( );
 Node nodeB = new Node( );
 Node nodeC = new Node( );
 Node nodeD = new Node( );
 Node nodeE = new Node( );

nodeA.data = "A"; nodeA.next = nodeB;
 nodeB.data = "B"; nodeB.next = nodeC;
 nodeC.data = "C"; nodeC.next = nodeD;

// Introduce a cycle in the linked-list

nodeD.data = "D"; nodeD.next = nodeB;

// Test for the presence of a cycle.

if(removeCycle( nodeA ) ) {
 System.out.println( "Cycle detected and removed!" );
 } else {
 System.out.println( "No cycle detected." );
 }

// Iterating through the linked list
 //
 for( Node cursor = nodeA; cursor != null; cursor = cursor.next ) {
 System.out.print( cursor.data + ( cursor.next != null ? " -> " : "" ) );
 //-> will be applied till cursor.next != null i.e. values a b c d are there else(:) " "
 }
 }
 }

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