detectloopLL.cpp

// 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 */
    new_node->next = (*head_ref);
 
    /* move the head to point to the new node */
    (*head_ref)    = new_node;
}
 
int detectloop(struct Node *list)
{
    struct Node *sp = list;
    struct Node *fp = list;
    while(sp && fp && fp->next)
    {
        sp = sp->next;
        fp = fp->next->next;
        if(sp == fp)
        {
            printf("there is a loop");
            return 1;
        }
    }
    return 0;
}
 
/* Drier 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;
    detectloop(head);
 
    return 0;
}