Loop through linked list Java

Example: Detect Loop in a LinkedList

class LinkedList { // create an object of Node class // represent the head of the linked list Node head; // static inner class static class Node { int value; // connect each node to next node Node next; Node(int d) { value = d; next = null; } } // check if loop is present public boolean checkLoop() { // create two references at start of LinkedList Node first = head; Node second = head; while(first != null && first.next !=null) { // move first reference by 2 nodes first = first.next.next; // move second reference by 1 node second = second.next; // if two references meet // then there is a loop if(first == second) { return true; } } return false; } public static void main(String[] args) { // create an object of LinkedList LinkedList linkedList = new LinkedList(); // assign values to each linked list node linkedList.head = new Node(1); Node second = new Node(2); Node third = new Node(3); Node fourth = new Node(4); // connect each node of linked list to next node linkedList.head.next = second; second.next = third; third.next = fourth; // make loop in LinkedList fourth.next = second; // printing node-value System.out.print("LinkedList: "); int i = 1; while (i <= 4) { System.out.print(linkedList.head.value + " "); linkedList.head = linkedList.head.next; i++; } // call method to check loop boolean loop = linkedList.checkLoop(); if(loop) { System.out.println("\nThere is a loop in LinkedList."); } else { System.out.println("\nThere is no loop in LinkedList."); } } }

Output

LinkedList: 1 2 3 4 There is a loop in LinkedList.

In the above example, we have implemented a LinkedList in Java. We have used Floyd's cycle finding algorithm to check if there is a loop in LinkedList.

Notice the code inside the checkLoop() method. Here, we have two variables named first and second that traverse the nodes in LinkedList.

• first - traverse with 2 nodes at single iteration
• second - traverse with 1 node at single iteration

Two nodes are traversing at different speeds. Hence, they will meet if there is a loop in LinkedList.