Detect and Remove Loop in a Linked List | GeeksforGeeks


 Detect and Remove Loop in a Linked List | GeeksforGeeks
Detect and Remove Loop in a Linked List 

This method is also dependent on Floyd’s Cycle detection algorithm.
1) Detect Loop using Floyd’s Cycle detection algo and get the pointer to a loop node.
2) Count the number of nodes in loop. Let the count be k.
3) Fix one pointer to the head and another to kth node from head.
4) Move both pointers at the same pace, they will meet at loop starting node.
5) Get pointer to the last node of loop and make next of it as NULL.


int detectAndRemoveLoop(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)
        {
            removeLoop(slow_p, list);
            /* Return 1 to indicate that loop is found */
            return 1;
        }
    }
    /* Return 0 to indeciate that ther is no loop*/
    return 0;
}
/* Function to remove loop.
 loop_node --> Pointer to one of the loop nodes
 head -->  Pointer to the start node of the linked list */
void removeLoop(struct node *loop_node, struct node *head)
{
    struct node *ptr1 = loop_node;
    struct node *ptr2 = loop_node;
    // Count the number of nodes in loop
    unsigned int k = 1, i;
    while (ptr1->next != ptr2)
    {
        ptr1 = ptr1->next;
        k++;
    }
    // Fix one pointer to head
    ptr1 = head;
    // And the other pointer to k nodes after head
    ptr2 = head;
    for(i = 0; i < k; i++)
      ptr2 = ptr2->next;
    /*  Move both pointers at the same pace,
      they will meet at loop starting node */
    while(ptr2 != ptr1)
    {
        ptr1 = ptr1->next;
        ptr2 = ptr2->next;
    }
    // Get pointer to the last node
    ptr2 = ptr2->next;
    while(ptr2->next != ptr1)
       ptr2 = ptr2->next;
    /* Set the next node of the loop ending node
      to fix the loop */
    ptr2->next = NULL;
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