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for this we should have the last pointer as well which point to the last node in the list.

how about circular list?

But with the first and last pointers, v can perform insertions @ both ends in O(1) time. If no last ptr is used, i think it takes O(n) time to insert at the end.(i.e to traverse the n nodes to reach to the last node).

for insert at begnning
insert_beg(struct node **q,int a)
{
struct node *temp;
temp=(struct node*)malloc(sizeof(struct node));
temp->data=a;
temp->next=*q;
*q=temp;
}

and insert at end is

insert_end(struct node **q,int a)
{
struct node *temp,r;
temp=*q;
while(temp->next!=NULL)
temp=temp>next;
r=malloc(sizeof(struct node));
r->data=a;
r->next=NULL:
temp->next=r;
}

place of r use *r