swap in doubly linked list

Question

I am trying to swap two nodes in a doubly linked list. Below is the part of program having swap function.

 int swap (int x, int y)
{
    struct node *temp = NU         


        

Answer 1

This can be compacted, but if you are having problems, it can help to spell it out in detail.

typedef struct node Node;

void link( Node* a, Node* b )
{
    a->next = b;
    b->prev = a;
}

void swap_nodes( Node* a, Node* b )
{
    if(a==b) return; // don't swap with yourself

    // handle adjacent nodes separately
    if( a->next == b )
    {
        Node* bef = a->prev;
        Node* aft = b->next;
        link( bef, b);    // link bef, b, a, aft
        link( b, a );
        link( a, aft );
    }
    else if( b->next == a )
    {
        Node* bef = b->prev;
        Node* aft = a->next;
        link( bef, a);   // link bef, a, b, aft
        link( a, b );
        link( b, aft );
    }
    else
    {
        Node* a_prv = a->prev;
        Node* a_nxt = a->next;
        Node* b_prv = b->prev;
        Node* b_nxt = b->next;

        link( a_prv, b ); link( b, a_nxt ); // links b in a's old position
        link( b_prv, a ); link( a, b_nxt ); // links a in b's old position
    }
}

Also note that your head node should never be null, it should be a sentry node that links to itself if your list is empty. This means that there are never a first node, nor a last, nor is the list ever empty. This removes a ton of special cases. See here