Cycle Detection HackerRank Solution

Cycle Detection HackerRank Solution

In this Cycle Detection HackerRank solution, A linked list is said to contain a cycle if any node is visited more than once while traversing the list. Given a pointer to the head of a linked list, determine if it contains a cycle. If it does, return . Otherwise, return .


 refers to the list of nodes 

The numbers shown are the node numbers, not their data values. There is no cycle in this list so return .

 refers to the list of nodes 

There is a cycle where node 3 points back to node 1, so return .

Function Description

Complete the has_cycle function in the editor below.

It has the following parameter:

  • SinglyLinkedListNode pointer head: a reference to the head of the list


  • int:  if there is a cycle or  if there is not

Note: If the list is empty,  will be null.

Input Format

The code stub reads from stdin and passes the appropriate argument to your function. The custom test cases format will not be described for this question due to its complexity. Expand the section for the main function and review the code if you would like to figure out how to create a custom case.


Sample Input

References to each of the following linked lists are passed as arguments to your function:

Sample Inputs

Sample Output



  1. The first list has no cycle, so return .
  2. The second list has a cycle, so return .

Cycle Detection HackerRank solution

I will Provide solution in Multiple programming languages for you. If you are not able to find the code in required language then please share in comments so that our team can help you.

Problem Solution in Python

def has_cycle(head):
    # dictionary of visits
    visits = {}
    # setting up head node
    node = head
    # visiting all nodes until duplicate found
    while node:
        node =
        if node in visits:
            return 1
            visits[node] = 1
    return 0

Problem Solution in C++

bool has_cycle(SinglyLinkedListNode* head) {
    unordered_set<SinglyLinkedListNode*> num_set;
    while (head) {        
        if (num_set.find(head) == num_set.end())
            return 1;
        head = head->next;
    return 0;

Problem Solution in Java

static boolean hasCycle(SinglyLinkedListNode head) {
        if(head == null){
            return false;
        Set<SinglyLinkedListNode> nodes = new HashSet<>();
        while(head != null){
                return true;
            head =;
        return false;


Problem Solution in C#

static bool hasCycle(SinglyLinkedListNode head) {
        var values = new HashSet<SinglyLinkedListNode>();
        while (head != null) {
            if (values.Contains(head))
                return true;
                head =;
        return false;    
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