# Reversing a LinkList in C?

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W linked list is a data structure consisting of a group of nodes which together represent a sequence. Under the simplest form, each node is composed of a data and a link to the next node in the sequence.

Reversing a link list can be done in two ways one by swapping the data values and another is by manipulating only the pointers, in this article I am discussing only the pointer manipulation mechanism in recursive and non-recursive ways.

Sample Code

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

int length= 0;

int list_size()
{
printf("\n No. of elements in the list: %d", length);
return length;
}

struct node * create_node(int data)
{
struct node *new;

new = (struct node*)malloc(sizeof(struct node));
new->next = null;
new->data = data;

length++;

return new;
}

{
struct node *new;
new = create_node(data);

{
}
else
{
}
}

void display_list()
{

{
printf("List is empty");
return;
}

{
}
}

void reverse_list()
{
struct node *p, *q, *r;

p = q = r = head;
p = p->next->next;
q = q->next;
r->next = NULL;
q->next = r;

while (p != NULL)
{
r = q;
q = p;
p = p->next;
q->next = r;
}
}

{

}
}
``````
posted Sep 2, 2014

## Related Articles

This program only first n elements of a linked list as compare to whole reverse as discussed in the http://tech.queryhome.com/57009/reversing-a-linklist-in-c . In this program we are manipulating the pointers only in place of data swap.

Sample Code

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

int length= 0;

int list_size()
{
printf("\n No. of elements in the list: %d", length);
return length;
}

struct node * create_node(int data)
{
struct node *new;

new = (struct node*)malloc(sizeof(struct node));
new->next = null;
new->data = data;

length++;

return new;
}

{
struct node *new;
new = create_node(data);

{
}
else
{
}
}

void display_list()
{

{
printf("List is empty");
return;
}

{
}
}

void reverse(struct node **head, int n)
{
struct node *p, *q, *r;

p = q = r = *head;

if (n > 0) // If N is zero then no need to reverse
{
p = p->next->next;
q = q->next;
r->next = NULL;
q->next = r;

while (n > 0 && p != NULL) // Run a loop for n times only
{
r = q;
q = p;
p = p->next;
q->next = r;
n--;
}
}
}
``````

What is Queue
Queue is a abstract data type with two operation add_data(enque) and get_data(deque) and provides the data in the form of its arrival or simply it maintains the first in first out machenism (FIFO). Data is always added to its top and retrieved from its rear.

Required Functions

``````int queue_size()
struct node * create_node(int data)
int get_data()
void traverse_queue()
``````

Sample Code

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

struct node *top, *rear;
int length= 0;

int queue_size()
{
printf("\n No. of elements in queue : %d", length);
return length;
}

struct node * create_node(int data)
{
struct node *new;

new = (struct node*)malloc(sizeof(struct node));
new->next = null;
new->data = data;

length++;

return new;
}

{
struct node *new;
new = create_node(data);

if ((top == NULL) && (rear == NULL))
{
top = new;
rear = new;
}
else
{
top->next = new;
top = new;
}
}

int get_data()
{
struct node *temp_rear;
int ret_val;

temp_rear = rear;

if (temp_rear == NULL)
{
printf("\n Error : Trying to pop from empty queue");
return -1;  // Assuming -1 as error
}
else
{
temp_rear = temp_rear->next;
}

printf("\n Popped value : %d", rear->data);
ret_val = rear->data;

free(rear);
rear = temp_rear;

length--;
return ret_val;
}

void traverse_queue()
{
struct node *temp_rear;

temp_rear = top;

if (temp_rear == NULL)
{
printf("Queue is empty");
return;
}

while (temp_rear != NULL)
{
printf("%d ", temp_rear->data);
temp_rear = temp_rear->next;
}
}
``````

W linked list is a data structure consisting of a group of nodes which together represent a sequence. Under the simplest form, each node is composed of a data and a link to the next node in the sequence.

As you can see in the first image that the last->next is null but if last->next is pointing to the head node is called circular link list. The main advantage id that if you have access to any node then you can get access of any node.

Necessary Functions

``````struct node * create_node(int data)
void insert_at_begining(int info)
void insert_at_position(int info, int position)
void delete_at_begining()
void delete_at_position(int position)
void search(int key)
void update(int olddata, int newdata)
void traverse()
void rev_traverse(struct node *p)
``````

Sample Code
C sample code someone can try C++ code...

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

/* We can have additional structure which can have head and last node is node pointer and
additionally can have length variable which contains the length of the link list */
struct node *head = NULL, *last = NULL;
int length = 0;

struct node * create_node(int data)
{
struct node *new;

new = (struct node*)malloc(sizeof(struct node));
new->next = null;
new->data = data;

length++;

return new;
}

void insert_at_begining(int info)
{
struct node *new;
new = create_node(info);

if (head == NULL && last == null)
{
}
else
{
}
}

void insert_at_position(int info, int position)
{
struct node *new, *ptr, *prevnode;
int len = 0, i;

new = create_node(info);

{
if (position == 1)
{
}
else
printf("\n empty linked list you cant insert at that particular position");
}
else
{
if (length < position)
printf("\n node cant be inserted as position is exceeding the linkedlist length");

else
{
for (ptr = head, i = 1; i <= length; i++)
{
prevnode = ptr;
ptr = ptr->next;
if (i == position-1)
{
prevnode->next = new;
new->next = ptr;
break;
}
}
}
}
}

void delete_at_begining()
{
struct node *y;

printf("\n List is empty");
else
{
x = last;
length--;
free(y);
}
}

void delete_at_position(int position)
{
int count = 0, i;
struct node *ptr, *temp, *prevnode;

printf("\n empty linked list you cant delete");
else
{
if (length < position)
printf("\n node cant be deleted at position as it is exceeding the linkedlist length");

else
{
for (ptr = head,i = 1; i <= length; i++)
{
prevnode = ptr;
ptr = ptr->next;
if (position == 1)
{
length--;
free(prevnode);
break;
}
else if (i == position - 1)
{
length--;
prevnode->next = ptr->next;
free(ptr);
break;
}
}
}
}
}

void search(int key)
{
int count = 0, i, f = 0;
struct node *ptr;

printf("\nlist is empty no elemnets in list to search");
else
{
for (ptr = head,i = 0; i < length; i++,ptr = ptr->next)
{
count++;
if (ptr->data == key)
{
printf("\n the value is found at position at %d", count);
f = 1;
}
}

if (f == 0)
}
}

void update(int olddata, int newdata)
{
int i, f = 0;
struct node *ptr;

printf("\n list is empty no elemnts for updation");
else
{
for (ptr = head, i = 0; i < length; ptr = ptr->next,i++)
{
if (ptr->data == olddata)
{
ptr->data = newdata;
printf("value is updated to %d", ptr->data);
f = 1;
}
}

if (f == 0)
printf("\n no such old value to be get updated");
}
}

void traverse()
{
printf("\n List is empty");
else
{
{
printf("%d->", x->data);
x = x->next;
}
printf("%d", x->data);
}
}

void rev_traverse(struct node *p)
{
int i = 0;

{
}
else
{
{
i = p->data;
rev_traverse(p->next);
printf(" %d", i);
}
{
printf(" %d", p->data);
}
}
}
``````

In a doubly linked list, each node contains, two links one is called next which points to the next node and one is called prev which points to the previous node in the sequence.

As you can see in the previous image that the head->prev is null and so the last->next but if head->prev is pointing to the last node and last->next is pointing to the head node is called doubly circular link list

Necessary Function
However it is dependent on how you like to manage the link list but the following function would be useful addition which can make any Doubly Link List useful in most of the situations -

``````struct node* create_node(int info)
void insert_at_first(int info)
void insert_at_last(int info)
void insert_at_position(int info, int position)
void sort_list()
void delete_node_position(int position)
void update(int olddata, int newdata)
void search(int key)
void display_from_begining()
void display_in_reverse()
``````

Sample Code
Here is the sample code (only the functions) written in C, any taker for C++

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

/* We can have additional structure which can have head and last node is node pointer and
additionally can have length variable which contains the length of the link list */
struct node *head = NULL, *last = NULL;
int length = 0;

struct node* create_node(int info)
{
struct node *new;
length++;
new = (struct node *)malloc(sizeof(struct node));
new->data = info;
new->next = NULL;
new->prev = NULL;
return new;
}

{
struct node *new;
new = create_node(info);
{
}
else
{
last->next = new;
new->prev = last;
last = new;
}
}

void insert_at_first(int info)
{
struct node *new;
new = create_node(info);
{
}
else
{
}
}

void insert_at_last(int info)
{
struct node *new;
new = create_node(info);
{
}
else
{
last->next = new;
new->prev = last;
last = new;
}
}

void insert_at_position(int info, int position)
{
struct node *new, *ptr, *prevnode;
int len = 0, i;

new = create_node(info);

{
if (position == 1)
{
}
else
printf("\n empty linked list you cant insert at that particular position");
}
else
{
if (length < position)
printf("\n node cant be inserted as position is exceeding the linkedlist length");

else
{
for (ptr = head, i = 1;i <= length;i++)
{
prevnode = ptr;
ptr = ptr->next;
if (i == position-1)
{
prevnode->next = new;
new->prev = prevnode;
new->next = ptr;
ptr->prev = new;
break;
}
}
}
}
}

void sort_list()
{
struct node *temp, *ptr;
int tempdata, i, j;

printf("\nlinked list is empty no elements to sort");
else
{
for (ptr = head,i = 0;i < length;ptr = ptr->next,i++)
{
for (temp = ptr->next,j=i;j<length;j++)
{
if (ptr->data > temp->data)
{
tempdata = ptr->data;
ptr->data = temp->data;
temp->data = tempdata;
}
}
}
for (ptr = head, i = 0;i < length;ptr = ptr->next,i++)
printf("\n%d", ptr->data);
}
}

void delete_node_position(int position)
{
int count = 0, i;
struct node *ptr, *temp, *prevnode;

printf("\n empty linked list you cant delete");
else
{
if (length < position)
printf("\n node cant be deleted at position as it is exceeding the linkedlist length");

else
{
for (ptr = head,i = 1;i <= length;i++)
{
prevnode = ptr;
ptr = ptr->next;
if (position == 1)
{
length--;
last->next = prevnode->next;
ptr->prev = prevnode->prev;
free(prevnode);
break;
}
else if (i == position - 1)
{
length--;
prevnode->next = ptr->next;
ptr->next->prev = prevnode;
free(ptr);
break;
}
}
}
}
}

void update(int olddata, int newdata)
{
int i, f = 0;
struct node *ptr;

printf("\n list is empty no elemnts for updation");
else
{
for (ptr = head, i = 0; i < length; ptr = ptr->next,i++)
{
if (ptr->data == olddata)
{
ptr->data = newdata;
printf("value is updated to %d", ptr->data);
f = 1;
}
}

if (f == 0)
printf("\n no such old value to be get updated");
}
}

void search(int key)
{
int count = 0, i, f = 0;
struct node *ptr;

printf("\nlist is empty no elemnets in list to search");
else
{
for (ptr = head,i = 0; i < length; i++,ptr = ptr->next)
{
count++;
if (ptr->data == key)
{
printf("\n the value is found at position at %d", count);
f = 1;
}
}

if (f == 0)
}
}

void display_from_begining()
{
int i;
struct node *ptr;

printf("\nlist is empty no elemnts to print");
else
{
printf("\n%d number of nodes are there", length);
for (ptr = head, i = 0;i < length;i++,ptr = ptr->next)
printf("\n %d", ptr->data);
}
}

void display_in_reverse()
{
int i;
struct node *ptr;

printf("\nlist is empty there are no elments");
else
{
for (ptr = last, i = 0;i < length;i++,ptr = ptr->prev)
{
printf("\n%d", ptr->data);
}
}
}
``````

Algorithm:
1. Traverse linked list using two pointers called slow pointer and fast pointer.
2. Move slow pointer by one in each iteration and fast pointer by two.
3. If these pointers meet then there is a loop. If pointers do not meet then linked list doesn’t have loop.

Sample Code

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

int length= 0;

int list_size()
{
printf("\n No. of elements in List : %d", length);
return length;
}

struct node * create_node(int data)
{
struct node *new;

new = (struct node*)malloc(sizeof(struct node));
new->next = null;
new->data = data;

length++;

return new;
}

{
struct node *new;
new = create_node(data);

{
}
else
{
}
}

void display_list()
{

{
printf("List is empty");
return;
}

{
}
}

{

while(slow_pointer && fast_pointer && fast_pointer->next )
{
slow_pointer = slow_pointer->next;
fast_pointer = fast_pointer->next->next;

if (slow_pointer == fast_pointer)
{
printf("Found Loop");
return 1;
}
}
return 0;
}
``````