C++ Program For Detecting Loop In A Linked List

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

The following are different ways of doing this. 

Solution 1: Hashing Approach:

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 the next of the current nodes 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 */
    new_node->next = (*head_ref);

    /* move the head to point to the new node */
    (*head_ref) = 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 this node is already present
        // in hashmap it means there is a cycle
        // (Because you we encountering the
        // node for the 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;
}

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

    push(&head, 20);
    push(&head, 4);
    push(&head, 15);
    push(&head, 10);

    /* Create a loop for testing */
    head->next->next->next->next = head;

    if (detectLoop(head))
        cout << "Loop found";
    else
        cout << "No Loop";

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

Output

Loop found

Complexity Analysis:

• Time complexity: O(n). 
  Only one traversal of the loop is needed.
• Auxiliary Space: O(n). 
  n is the space required to store the value in hashmap.

Solution 2: This problem can be solved without hashmap by modifying the linked list data structure. 
Approach: This solution requires modifications to the 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 a 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.

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;
    int flag;
};

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;

    new_node->flag = 0;

    /* link the old list off the new node */
    new_node->next = (*head_ref);

    /* move the head to point to the new node */
    (*head_ref) = new_node;
}

// Returns true if there is a loop in linked list
// else returns false.
bool detectLoop(struct Node* h)
{
    while (h != NULL) {
        // If this node is already traverse
        // it means there is a cycle
        // (Because you we encountering the
        // node for the second time).
        if (h->flag == 1)
            return true;

        // If we are seeing the node for
        // the first time, mark its flag as 1
        h->flag = 1;

        h = h->next;
    }

    return false;
}

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

    push(&head, 20);
    push(&head, 4);
    push(&head, 15);
    push(&head, 10);

    /* Create a loop for testing */
    head->next->next->next->next = head;

    if (detectLoop(head))
        cout << "Loop found";
    else
        cout << "No Loop";

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

Output

Loop found

Complexity Analysis:

• Time complexity:O(n). 
  Only one traversal of the loop is needed.
• Auxiliary Space:O(1). 
  No extra space is needed.

Solution 3: Floyd’s Cycle-Finding Algorithm
Approach: This is the fastest method and has been described below:  

• Traverse linked list using two pointers.
• Move one pointer(slow_p) by one and another pointer(fast_p) by two.
• If these pointers meet at the same node then there is a loop. If pointers do not meet then linked list doesn’t have a loop.

The below image shows how the detectloop function works in the code:

Implementation of Floyd’s Cycle-Finding Algorithm:  

C++

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

/* Link list node */
class Node {
public:
    int data;
    Node* next;
};

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

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

    /* link the old list off the new node */
    new_node->next = (*head_ref);

    /* move the head to point to the new node */
    (*head_ref) = new_node;
}

int detectLoop(Node* list)
{
    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) {
            return 1;
        }
    }
    return 0;
}

/* Driver code*/
int main()
{
    /* Start with the empty list */
    Node* head = NULL;

    push(&head, 20);
    push(&head, 4);
    push(&head, 15);
    push(&head, 10);

    /* Create a loop for testing */
    head->next->next->next->next = head;
    if (detectLoop(head))
        cout << "Loop found";
    else
        cout << "No Loop";
    return 0;
}

// This is code is contributed by rathbhupendra

Output

Loop found

Complexity Analysis:

• Time complexity: O(n). 
  Only one traversal of the loop is needed.
• Auxiliary Space:O(1). 
  There is no space required.

How does above algorithm work?
Please See : How does Floyd’s slow and fast pointers approach work?
https://www.youtube.com/watch?v=Aup0kOWoMVg 

Solution 4: Marking visited nodes without modifying the linked list data structure
In this method, a temporary node is created. The next pointer of each node that is traversed is made to point to this temporary node. This way we are using the next pointer of a node as a flag to indicate whether the node has been traversed or not. Every node is checked to see if the next is pointing to a temporary node or not. In the case of the first node of the loop, the second time we traverse it this condition will be true, hence we find that loop exists. If we come across a node that points to null then the loop doesn’t exist.

Below is the implementation of the above approach:

C++

// C++ program to return first node of loop
#include <bits/stdc++.h>
using namespace std;

struct Node {
    int key;
    struct Node* next;
};

Node* newNode(int key)
{
    Node* temp = new Node;
    temp->key = key;
    temp->next = NULL;
    return temp;
}

// A utility function to print a linked list
void printList(Node* head)
{
    while (head != NULL) {
        cout << head->key << " ";
        head = head->next;
    }
    cout << endl;
}

// Function to detect first node of loop
// in a linked list that may contain loop
bool detectLoop(Node* head)
{

    // Create a temporary node
    Node* temp = new Node;
    while (head != NULL) {

        // This condition is for the case
        // when there is no loop
        if (head->next == NULL) {
            return false;
        }

        // Check if next is already
        // pointing to temp
        if (head->next == temp) {
            return true;
        }

        // Store the pointer to the next node
        // in order to get to it in the next step
        Node* nex = head->next;

        // Make next point to temp
        head->next = temp;

        // Get to the next node in the list
        head = nex;
    }

    return false;
}

/* Driver program to test above function*/
int main()
{
    Node* head = newNode(1);
    head->next = newNode(2);
    head->next->next = newNode(3);
    head->next->next->next = newNode(4);
    head->next->next->next->next = newNode(5);

    /* Create a loop for testing(5 is pointing to 3) */
    head->next->next->next->next->next = head->next->next;

    bool found = detectLoop(head);
    if (found)
        cout << "Loop Found";
    else
        cout << "No Loop";

    return 0;
}

Output

Loop Found

Complexity Analysis:

• Time complexity: O(n). 
  Only one traversal of the loop is needed.
• Auxiliary Space: O(1). 
  There is no space required.

Solution 5: Store length

In this method, two pointers are created, first (always points to head) and last. Each time the last pointer moves we calculate no of nodes in between first and last and check whether the current no of nodes > previous no of nodes, if yes we proceed by moving last pointer else it means we’ve reached the end of the loop, so we return output accordingly.

C++

// C++ program to return first node of loop
#include <bits/stdc++.h>
using namespace std;

struct Node {
    int key;
    struct Node* next;
};

Node* newNode(int key)
{
    Node* temp = new Node;
    temp->key = key;
    temp->next = NULL;
    return temp;
}

// A utility function to print a linked list
void printList(Node* head)
{
    while (head != NULL) {
        cout << head->key << " ";
        head = head->next;
    }
    cout << endl;
}

/*returns distance between first and last node every time
 * last node moves forwards*/
int distance(Node* first, Node* last)
{
    /*counts no of nodes between first and last*/
    int counter = 0;

    Node* curr;
    curr = first;

    while (curr != last) {
        counter += 1;
        curr = curr->next;
    }

    return counter + 1;
}

// Function to detect first node of loop
// in a linked list that may contain loop
bool detectLoop(Node* head)
{

    // Create a temporary node
    Node* temp = new Node;

    Node *first, *last;

    /*first always points to head*/
    first = head;
    /*last pointer initially points to head*/
    last = head;

    /*current_length stores no of nodes between current
     * position of first and last*/
    int current_length = 0;

    /*current_length stores no of nodes between previous
     * position of first and last*/
    int prev_length = -1;

    while (current_length > prev_length && last != NULL) {
        // set prev_length to current length then update the
        // current length
          prev_length = current_length;
        // distance is calculated
        current_length = distance(first, last);
        // last node points the next node
        last = last->next;
    }

    if (last == NULL) {
        return false;
    }
    else {
        return true;
    }
}

/* Driver program to test above function*/
int main()
{
    Node* head = newNode(1);
    head->next = newNode(2);
    head->next->next = newNode(3);
    head->next->next->next = newNode(4);
    head->next->next->next->next = newNode(5);

    /* Create a loop for testing(5 is pointing to 3) */
    head->next->next->next->next->next = head->next->next;

    bool found = detectLoop(head);
    if (found)
        cout << "Loop Found";
    else
        cout << "No Loop Found";

    return 0;
}

Output

Loop Found

Complexity Analysis:

• Time complexity: O(n²)
• Auxiliary Space: O(1)

Another Approach:

1. This is the simplest approach of the given problem, the only thing we have to do is to assign a new value to each data of node in the linked list which is not in the range given.
2. Example suppose (1 <= Data on Node <= 10^3) then after visiting node assign the data as -1 as it is out of the given range.

Follow the code given below for a better understanding:

C++

// C++ program to return first node of loop
#include <bits/stdc++.h>
using namespace std;

struct Node {
    int key;
    struct Node* next;
};

Node* newNode(int key)
{
    Node* temp = new Node;
    temp->key = key;
    temp->next = NULL;
    return temp;
}

// Function to detect first node of loop
// in a linked list that may contain loop
bool detectLoop(Node* head)
{

    // If the head is null we will return false
    if (!head)
        return 0;
    else {

        // Traversing the linked list
        // for detecting loop
        while (head) {
            // If loop found
            if (head->key == -1) {
                return true;
            }

            // Changing the data of visited node to any
            // value which is outside the given range here it
            // is supposed the given range is (1 <= Data on
            // Node <= 10^3)
            else {
                head->key = -1;
                head = head->next;
            }
        }
        // If loop not found return false
        return 0;
    }
}

/* Driver program to test above function*/
int main()
{
    Node* head = newNode(1);
    head->next = newNode(2);
    head->next->next = newNode(3);
    head->next->next->next = newNode(4);
    head->next->next->next->next = newNode(5);

    /* Create a loop for testing(5 is pointing to 3) */
    head->next->next->next->next->next = head->next->next;

    bool found = detectLoop(head);
    cout << found << endl;
    return 0;
}

Output

1

Time Complexity: O(N)

Auxiliary Space: O(1)

Please refer complete article on Detect loop in a linked list for more details!