How to detect a cycle in a linked list ?

Editor — Tue, 13 Jul 2021 16:48:05 +0000

How to detect a cycle in a linked list ?

A linked list is said to contain a cycle if any node is visited more than once.

Method 1 – Using Hashing

Traverse the given list.
Insert each encountered node into a hash table.
If the current already present ,that means the cycle is present or not.

Sample code

#include 
#include 
using namespace std;

// Data Structure to store a linked list node
struct Node
{
    int dt;
    Node* next;
};

// Helper function to create a new node with the given data and
// pushes it onto the front of the list
void push(Node*& headRef, int dt)
{
    // create a new linked list node from heap
    Node* newNode = new Node;

    newNode->dt = dt;
    newNode->next = headRef;
    headRef = newNode;
}

// Function to detect Cycle in a linked list using Hashing
bool detectcyc(Node *head)
{
    Node *current = head;
    unordered_set set;

    // traverse the list
    while (current)
    {
        // return false if we already have seen this node before
        if (set.find(current) != set.end())
            return true;

        // insert current node into the set
        set.insert(current);

        // move to the next node
        current = current->next;
    }

    // we reach here if list does not contain any cycle
    return false;
}

// Detect Cycle in a linked list
int main()
{
    // input keys
    int k[] = { 1, 2, 3, 4, 5 };
    int n = sizeof(k) / sizeof(k[0]);

    Node* head = nullptr;
    for (int i = n - 1; i >= 0; i--)
        push(head, k[i]);

    // insert cycle
    head->next->next->next->next->next = head->next->next;

    if (detectcyc(head))
        cout << "Cycle Found";
    else
        cout << "No Cycle Found";

    return 0;
}

Output

Cycle Found

Method 2 – Floyd’s Cycle Detection Algorithm

Use two pointers, which move through the sequence at different speed.
The fast pointer moves twice as quickly as the slow pointer.
Here the distance between the two pointers increased by 1 at each step.
If these pointers meet at same node then there is a loop else linked list doesn’t have loop.

Sample Code

#include 
#include 
using namespace std;

// Data Structure to store a linked list node
struct Node
{
    int dt;
    Node* next;
};

// Helper function to create a new node with the given data and
// pushes it onto the front of the list
void push(Node*& headRef, int dt)
{
    // create a new linked list node from heap
    Node* nNode = new Node;

    nNode->dt = dt;
    nNode->next = headRef;
    headRef = nNode;
}

// Function to detect Cycle in a linked list using
// Floyd’s Cycle Detection Algorithm
bool dcycle(Node *head)
{
    // take two pointers - slow and fast
    Node *slow = head, *fast = head;

    while (fast && fast->next)
    {
        // move slow by one pointer
        slow = slow->next;

        // move fast by two pointers
        fast = fast->next->next;

        // if they meet any any node, linked list contains a cycle
        if (slow == fast)
            return true;
    }

    // we reach here if slow & fast pointer do not meet
    return false;
}

// Detect Cycle in a linked list using Floyd’s Cycle Detection Algorithm
int main()
{
    // input keys
    int k[] = { 1, 2, 3, 4, 5 };
    int n = sizeof(k) / sizeof(k[0]);

    Node* head = nullptr;
    for (int i = n - 1; i >= 0; i--)
        push(head, k[i]);

    // insert cycle
    head->next->next->next->next->next = head->next->next;

    if (dcycle(head))
        cout << "Cycle Found";
    else
        cout << "No Cycle Found";

    return 0;
}

Output

Cycle Found

cycle detection solution - Wikitechy

How to detect a cycle in a linked list ?

How to detect a cycle in a linked list ?

Method 1 – Using Hashing

Sample code

Output

Method 2 – Floyd’s Cycle Detection Algorithm

Sample Code

Output