Reversing Link List in group of n nodes without using any other data type in O(n) time

Some ways of reversing a link list using other data types are-
1.Use the stack. We can push all the elements on to the stack and then poop out each element. 
2.Similarly, we can also create a new link list. Starting from the head of the current list, we can create  new list by adding nodes in the front of the new list.

A link list can be reversed without using any other data type by using three pointers as shown in the code below.

//created by Aditya Vijayvergia

//uploaded on thecodingedge.blogspot.com

#include<stdio.h>

#include<conio.h>

struct node

{

  int data;

  struct node* next;

}*head;

typedef struct node node;

node* rev(int n)

{

  node *p,*q,*r,*t2,*t1;

  int c=0,start=1;

  t2=head;

  t1=NULL;

  p=head;

  q=head->next;

  r=q->next;

  while(q!=NULL)

  {

    //printf("c<n-1\n\t%d",p->data);

    //getch();

    if(c==n-1)

    {

      //printf("c=n-1");

      //getch();

      c=0;

      if(t1!=NULL)

    t1->next=p;

      t1=t2;

      t2=q;

      if(start==1)

      {

    start=0;

    head=p;

      }

      p=q;

      q=p->next;

      r=q->next;

    }

    q->next=p;

    p=q;

    q=r;

    r=r->next;

    c++;

  }

  t1->next=p;

  t2->next=NULL;

  return head;

}

void main()

{

  node *q;

  node* p;

  int i;

  clrscr();

  head=(node*)malloc(sizeof(node));

  p=head;

  //q->next=NULL;

  head->data=1;

  //p->next=NULL;

  for(i=2;i<15;i++)

  {

    q=(node*)malloc(sizeof(node));

    q->next=NULL;

    q->data=i;

    p->next=q;

    p=p->next;

  }

  p=head;

  printf("Initial link list is:\n");

  while(p!=NULL)

  {

    printf("%d->",p->data);

    p=p->next;

  }

  printf("\n\n\n\nUsing n=3\n");

  head=rev(6);

  p=head;

  printf("\n\n\nReversed link list is:\n");

  while(p!=NULL)

  {

    printf("%d->",p->data);

    p=p->next;

  }

  getch();

}

The output of the program is given below.