C Exercises: Remove a loop in a singly linked list

Last update on August 02 2025 12:54:48 (UTC/GMT +8 hours)

14. Loop Detection Challenges

Write a C program to detect and remove a loop in a singly linked list.

Sample Solution:

C Code:

#include<stdio.h>
#include <stdlib.h>

// Node structure for the linked list
struct Node {
    int data;
    struct Node* next;
};

// Function to create a new node
struct Node* newNode(int data) {
    struct Node* node = (struct Node*) malloc(sizeof(struct Node));
    node->data = data;
    node->next = NULL;
    return node;
}

// Function to detect and remove loop in a linked list
void detect_and_remove_Loop(struct Node* head) {
    struct Node* slow = head;
    struct Node* fast = head;

    // Use Floyd's cycle-finding algorithm to detect a loop
    while (fast && fast->next) {
        slow = slow->next;
        fast = fast->next->next;
        if (slow == fast) break; // If a loop is detected, break the loop
    }

    // If a loop is detected
    if (slow == fast) {
        slow = head;
        while (slow->next != fast->next) {
            slow = slow->next;
            fast = fast->next;
        }

        // Break the loop by setting the next pointer of the loop node to NULL
        fast->next = NULL;
    }
}

// Function to print a linked list
void displayList(struct Node* head) {
    while (head) {
        printf("%d ", head->data);
        head = head->next;
    }
    printf("\n");
}

int main() {
    // Creating a singly linked list
    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);

    printf("Original singly linked list:\n");
    displayList(head);

    // Creating a loop in the linked list
    printf("\nCreate the loop:\n");
	head->next->next->next->next->next = head->next->next;

    // Displaying the loop (this line is commented out to avoid infinite loop in printing)
    //printf("Following statement display the loop:\n");
    //displayList(head);

    // Removing the loop from the linked list
	printf("\n\nAfter removing the said loop:\n");
    detect_and_remove_Loop(head);
    displayList(head);
    return 0;
}

Sample Output:

Original singly linked list:
1 2 3 4 5 

Create the loop:
Following statement display the loop:
displayList(head);

After removing the said loop:
1 2 3 4 5

Flowchart :

For more Practice: Solve these Related Problems:

Write a C program to detect a loop in a singly linked list using Floyd’s cycle-finding algorithm and remove it.
Write a C program to remove a loop in a linked list by storing visited nodes in a hash table.
Write a C program to detect a loop in a linked list by counting node visits and then remove it.
Write a C program to check for the presence of multiple loops in a linked list and remove them safely.

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