Cycle Detection


Problem Statement :


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 1. Otherwise, return 0.

Example

  head refers 1 -> 2 -> 3 -> NUL

The numbers shown are the node numbers, not their data values. There is no cycle in this list so return 0.

 head refers 1 -> 2 -> 3 -> 1 -> NULL

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

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
Returns

int: 1 if there is a cycle or 0 if there is not
Note: If the list is empty, head 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.


Solution :



title-img 


                            Solution in C :

In C++ :


/*
  Detect loop in a linked list 
  List could be empty also
  Node is defined as 
  struct Node
  {
     int data;
     struct Node *next;
  }
*/
int HasCycle(Node* head)
{
   // Complete this function
   // Do not write the main method
    Node  *slowp = head, *fastp = head;
 
    while (slowp && fastp && fastp->next)
    {
        slowp = slowp->next;
        fastp  = fastp->next->next;
 
        if (slowp == fastp)
        {
            return 1;
        }
    }

    return 0;
}




In Java :



/*
  Insert Node at the end of a linked list 
  head pointer input could be NULL as well for empty list
  Node is defined as 
  class Node {
     int data;
     Node next;
  }
*/
int HasCycle(Node head) {
    
    if(head==null)
        return 0;
    
    if(head.next==head)
        return 1;
    
    Node slow=head;
    Node fast=head;
    
    while(fast!=null)
        {
        
         fast=fast.next;
         if(fast!=null)
             fast=fast.next;
         else
             return 0;
        
         slow=slow.next;
        
        if(slow==fast)
            return 1;
        
    }
    return 0;
    
    
    
}


In C :


// Complete the has_cycle function below.

/*
 * For your reference:
 *
 * SinglyLinkedListNode {
 *     int data;
 *     SinglyLinkedListNode* next;
 * };
 *
 */
bool has_cycle(SinglyLinkedListNode* head) {

    struct SinglyLinkedListNode* slow=head, * fast=head;
    while((slow!=NULL)&&(fast!=NULL)&&(fast->next!=NULL))
    {
        slow=slow->next;
        fast=fast->next->next;
        if(slow==fast)
            return 1;
    }
    return 0;

}



In python3 : 



"""
 Check if linked list has cycle
 head could be None as well for empty list
 Node is defined as
 
 class Node(object):
 
   def __init__(self, data=None, next_node=None):
       self.data = data
       self.next = next_node

 return 0 if no cycle else return 1
"""

def HasCycle(head):
    slow = head
    fast = head.next
    count = 0
    while slow and fast and count==0:
        if slow == fast:
            count=1
        else:
            slow= slow.next
            fast = fast.next.next
    if fast !=None or count == 1:
        return(1)
    else:
        return(0)








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