# Detect loop in a linked list

Given a linked list, check if the the linked list has loop or not. Below diagram shows a linked list with a loop.

## Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

Following are different ways of doing this
Use Hashing:
Traverse the list one by one and keep putting the node addresses in a Hash Table. At any point, if NULL is reached then return false and if next of current node points to any of the previously stored nodes in Hash then return true.

## C++

```// C++ program to detect loop in a linked list
#include<bits/stdc++.h>
using namespace std;

/* Link list node */
struct Node
{
int data;
struct Node* next;
};

void push(struct Node** head_ref, int new_data)
{
/* allocate node */
struct Node* new_node = new Node;

/* put in the data  */
new_node->data  = new_data;

/* link the old list off the new node */

/* move the head to point to the new node */
}

// Returns true if there is a loop in linked list
// else returns false.
bool detectLoop(struct Node *h)
{
unordered_set<Node *> s;
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.find(h) != s.end())
return true;

// If we are seeing the node for
// the first time, insert it in hash
s.insert(h);

h = h->next;
}

return false;
}

/* Drier program to test above function*/
int main()
{
/* Start with the empty list */
struct Node* head = NULL;

/* Create a loop for testing */

cout << "Loop found";
else
cout << "No Loop";

return 0;
}
// This code is contributed by Geetanjali
```

## Java

```// 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 */

/* 4. Move the head to point to 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

h = h.next;
}

return false;
}

/* Driver program to test above function */
public static void main(String[] args)
{

llist.push(20);
llist.push(4);
llist.push(15);
llist.push(10);

/*Create loop for testing */

System.out.println("Loop found");
else
System.out.println("No Loop");

}
}

// This code is contributed by Arnav Kr. Mandal.
```

Output :
```Loop Found
```

Mark Visited Nodes:
This solution requires modifications to basic linked list data structure.  Have a visited flag with each node.  Traverse the linked list and keep marking visited nodes.  If you see a visited node again then there is a loop. This solution works in O(n) but requires additional information with each node.
A variation of this solution that doesn’t require modification to basic data structure can be implemented using hash.  Just store the addresses of visited nodes in a hash and if you see an address that already exists in hash then there is a loop.

Floyd’s Cycle-Finding Algorithm:
This is the fastest method. Traverse linked list using two pointers.  Move one pointer by one and other pointer by two.  If these pointers meet at some node then there is a loop.  If pointers do not meet then linked list doesn’t have loop.

Implementation of Floyd’s Cycle-Finding Algorithm:

## C/C++

```// C program to detect loop in a linked list
#include<stdio.h>
#include<stdlib.h>

/* Link list node */
struct Node
{
int data;
struct Node* next;
};

void push(struct Node** head_ref, int new_data)
{
/* allocate node */
struct Node* new_node =
(struct Node*) malloc(sizeof(struct Node));

/* put in the data  */
new_node->data  = new_data;

/* link the old list off the new node */

/* move the head to point to the new node */
}

int detectloop(struct Node *list)
{
struct Node  *slow_p = list, *fast_p = list;

while (slow_p && fast_p && fast_p->next )
{
slow_p = slow_p->next;
fast_p  = fast_p->next->next;
if (slow_p == fast_p)
{
printf("Found Loop");
return 1;
}
}
return 0;
}

/* Drier program to test above function*/
int main()
{
/* Start with the empty list */
struct Node* head = NULL;

/* Create a loop for testing */

return 0;
}
```

## Java

```// Java program to detect loop in a linked list
{

/* Linked list Node*/
class Node
{
int data;
Node next;
Node(int d) {data = d; next = null; }
}

/* Inserts a new Node at front of the list. */
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 */

/* 4. Move the head to point to new Node */
}

int detectLoop()
{
Node slow_p = head, fast_p = head;
while (slow_p != null && fast_p != null && fast_p.next != null) {
slow_p = slow_p.next;
fast_p = fast_p.next.next;
if (slow_p == fast_p) {
System.out.println("Found loop");
return 1;
}
}
return 0;
}

/* Drier program to test above functions */
public static void main(String args[])
{

llist.push(20);
llist.push(4);
llist.push(15);
llist.push(10);

/*Create loop for testing */

llist.detectLoop();
}
}
/* This code is contributed by Rajat Mishra. */
```

## Python

```
# Python program to detect loop in the linked list

# Node class
class Node:

# Constructor to initialize the node object
def __init__(self, data):
self.data = data
self.next = None

# Function to initialize head
def __init__(self):

# Function to insert a new node at the beginning
def push(self, new_data):
new_node = Node(new_data)

# Utility function to prit the linked LinkedList
def printList(self):
while(temp):
print temp.data,
temp = temp.next

def detectLoop(self):
while(slow_p and fast_p and fast_p.next):
slow_p = slow_p.next
fast_p = fast_p.next.next
if slow_p == fast_p:
print "Found Loop"
return

# Driver program for testing
llist.push(20)
llist.push(4)
llist.push(15)
llist.push(10)

# Create a loop for testing
llist.detectLoop()

# This code is contributed by Nikhil Kumar Singh(nickzuck_007)

```

Output:
`Found loop`

Time Complexity: O(n)
Auxiliary Space: O(1)

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