Swap nodes in a singly-linked list

Question

I am trying to swap two nodes. For example if the nodes are a and b I am passing the pointers
(a-1)->next and (b-1)->n

Answer 1

I was hoping for cut-and-paste code, but didn't find it. If anyone is still interested, this is the answer. I did the brute force analysis of all 3 cases.

// swaps nodes at locations *sp and *tp
struct stack *s = *sp; // store locations to prevent overwritten bugs
struct stack *t = *tp;
if(&t->next == sp) { // order u (tp)-> t (sp)-> s -> ...
    t->next = s->next;
    s->next = t;
    *tp = s;
} else if(&s->next == tp) { // order u (sp)-> s (tp)-> t -> ...
    s->next = t->next;
    t->next = s;
    *sp = t;
} else { // disconnected u (sp)->s -> ..., v (tp)->t -> ...
    struct stack *next = s->next;
    s->next = t->next;
    t->next = next;
    *sp = t;
    *tp = s;
}             
// If you track the end of your list, it be the one pointing to NULL.                                           
if(s->next == NULL) {                                  
    end = &s->next;                                 
} else if(t->next == NULL) {                           
    end = &t->next;                                 
}

Note: This code works if sp == tp, but assumes you don't have the pathological case where BOTH &s->next == tp && &t->next == sp (starting with cycle).