Reverse adjacent nodes in a linked list

Reverse the adjacent nodes in a linked list If the nodes of a linked list are as follows 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 -> 9 then after reverse they should be 2 -> 1 -> 4 -> 3 Here the number of entries are odd hence the last link is not reversed. If number of nodes are even then last node should also be reversed.

node * swapAdjacent(node * head){
    node *a,*b,*c,*d;
    a = head;
    if(a == null)
        return a;
    b = a->next;
    if(b == null)
        return a;
    c = b->next;
    if(c == null){
        b->next = a;
        a->next = c;
        return b;
    }
    d = c->next;

    //now all a,b,c are non null. size of list is  >=3
    node * newHead = b;
    while(b!= null){
        b->next = a;
        if(d != null)
            a->next = d;
        else
            a->next = c;
        a = c;
        b = d;
        c = (b!=null)?b->next : null;
        d = (c!=null)?c->next : null;
    }
    return newHead;
}