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. 

Solution: 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 <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 *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 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))
        printf("Loop found");
    else
        printf("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.

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

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