Find length of loop in linked list

Write a function detectAndCountLoop() that checks whether a given Linked List contains loop and if loop is present then returns count of nodes in loop. For example, loop is present in below linked list and length of loop is 4. If loop is not present, then function should return 0.

We know that Floyd’s Cycle detection algorithm terminates when fast and slow pointers meet at a common point. We also know that this common point is one of the loop nodes (2 or 3 or 4 or 5 in the above diagram). We store the address of this in a pointer variable say ptr2. Then we start from the head of the Linked List and check for nodes one by one if they are reachable from ptr2. When we find a node that is reachable, we know that this node is the starting node of the loop in Linked List and we can get pointer to the previous of this node.

// C program to count number of nodes
// in loop in a linked list if loop is
// present

/* Link list node */
struct Node
    int data;
    struct Node* next;

// Returns count of nodes present in loop.
int countNodes(struct Node *n)
   int res = 1;
   struct Node *temp = n;
   while (temp->next != n)
      temp = temp->next;
   return res;

/* This function detects and counts loop
   nodes in the list. If loop is not there
   in then returns 0 */
int countNodesinLoop(struct Node *list)
    struct 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 and fast_p meet at some point
           then there is a loop */
        if (slow_p == fast_p)
            return countNodes(slow_p);

    /* Return 0 to indeciate that ther is no loop*/
    return 0;

struct Node *newNode(int key)
    struct Node *temp =
        (struct Node*)malloc(sizeof(struct Node));
    temp->data = key;
    temp->next = NULL;
    return temp;

/* Drier program to test above function*/
int main()
    struct 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 */
    head->next->next->next->next->next = head->next;

    printf("%d \n", countNodesinLoop(head));

    return 0;

Output :


Related Articles :
Detect loop in a linked list
Detect and Remove Loop in a Linked List

This article is contributed by Shubham Gupta. If you like GeeksforGeeks and would like to contribute, you can also write an article using or mail your article to See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

Please write to us at to report any issue with the above content.

Recommended Posts:

2.2 Average Difficulty : 2.2/5.0
Based on 16 vote(s)