Detect a loop in linked list (#2346)

Co-authored-by: Andrii Siriak <siryaka@gmail.com>

DataStructures/Lists/detect_and_create_loop.jav

@@ -0,0 +1,107 @@
+import java.util.*;
+import java.util.Scanner;
+public class LinkedList {
+
+    static Node head; // head of list
+
+    static class Node // Linked list Node
+    {
+        int data; //to store value
+        Node next; //pointer
+        Node(int d) {
+            data = d;
+            next = null;
+        }
+    }
+
+    static int countNodes(Node ptr) //Function to count the number of nodes present
+    {
+        int count = 0;
+        while (ptr != null) {
+            ptr = ptr.next;
+            count++;
+        }
+        return count;
+    }
+
+    static public void push(int new_data) // Function to inserts a new Node at front of the list 
+    {
+
+        Node new_node = new Node(new_data); //Allocate a pointer/node and store the data
+
+        new_node.next = head; // make next of new Node as head 
+
+        head = new_node; // Move the head to point to new Node.
+    }
+
+    static void printList(Node head, int total_nodes) //function to traverse through the list and print all data values
+    {
+        Node curr = head;
+        int count = 0;
+        while (count < total_nodes) {
+            count++;
+            System.out.print(curr.data + " ");
+            curr = curr.next;
+        }
+    }
+
+    static Node makeloop(Node head_ref, int k) {
+        Node temp = head_ref;
+        int count = 1;
+        while (count < k) //traverrse the list till point is found
+        {
+            temp = temp.next;
+            count++;
+        }
+
+        Node connected_point = temp;
+
+        while (temp.next != null) // traverse remaining nodes
+            temp = temp.next;
+
+        temp.next = connected_point; //connect last node to k-th element
+        return head_ref;
+    }
+
+    static boolean detectLoop(Node h) //Function to detect loop, retuens true if loop is in linked list else returns false.
+    {
+        HashSet < Node > traverse = new HashSet < Node > ();
+        while (n != null) {
+
+            if (traverse.contains(n)) //if the hash a;ready contains a record of the node encountered true is returned as a loop isdetected
+                return true;
+
+            traverse.add(n);
+            n = n.next;
+        }
+        return false;
+    }
+
+    public static void main(String[] args) {
+        LinkedList l = new LinkedList();
+
+        Scanner sc = new Scanner(System.in);
+
+        print("Enter elements in the list, to stop entering press any alphabetical key");
+        while (true) {
+            try {
+                i = sc.nextInt();
+                l.push(i);
+            } catch (Exception e) {
+                System.out.println("Creating loop for run");
+            }
+        }
+        System.out.println("Enter the location to generate loop");
+        int k = sc.nextInt()
+        System.out.print("Given list");
+        printList(head, total_nodes);
+        head = makeloop(head, k);
+        System.out.print("Modified list with loop");
+        printList(head, total_nodes);
+
+        if (detectLoop(head))
+            System.out.println("Loop found");
+        else
+            System.out.println("No Loop");
+    }
+}