WAP to Remove Loop From Doubly Linked List Efficiently

Algorithm


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.

Note:One Test case Not Covered That I Will Do Later it Involve Prev Pointer




#include
#include

struct node
{
int data;
struct node *next;
struct node *prev;
};

/* Function to remove loop. */
int removeLoop(struct node *, struct node *);

/* This function detects and removes loop in the list
If loop was there in the list then it returns 1,
otherwise returns 0 */
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 */
int 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;
}

/* UTILITY FUNCTIONS */
/* Given a reference (pointer to pointer) to the head
of a list and an int, pushes a new node on the front
of the list. */
/* UTILITY FUNCTIONS */
/* Function to insert a node at the beginging of the Doubly Linked List */
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;

/* since we are adding at the begining,
prev is always NULL */
new_node->prev = NULL;

/* link the old list off the new node */
new_node->next = (*head_ref);

/* change prev of head node to new node */
if((*head_ref) != NULL)
(*head_ref)->prev = new_node ;

/* move the head to point to the new node */
(*head_ref) = new_node;
}


/* Function to print linked list */
void printList(struct node *node)
{
while(node != NULL)
{
printf("%d ", node->data);
node = node->next;
}
}

/* Drier program to test above function*/
int main()
{
/* Start with the empty list */
struct node* head = NULL;

push(&head, 10);
push(&head, 4);
push(&head, 15);
push(&head, 20);
push(&head, 50);

/* Create a loop for testing */
head->next->next->next->next->next = head->next->next;

detectAndRemoveLoop(head);

printf("Linked List after removing loop \n");
printList(head);

getchar();
return 0;
}

TC O(n)
SC O(1)
Run Here https://ideone.com/sHvMp