Question

I’m working on a university assignment. I am trying to write a linked-list sort in C. I am not allowed to swap values – only pointers.

Here is my sorting function:

struct node *sort_list(struct node *head) {

bool swapped ;
struct node *cur = head, *first ;

if ( head == NULL || head->next == NULL ) return head ;
else {
    do {
        swapped = false ;
        while ( cur != NULL && cur->next != NULL ){
            if (cur->value > cur->next->value){
                cur = swap_with_next( cur ) ;
                swapped = true ;
            }
            cur = cur->next ;
        }   
    } while (swapped == true) ;

}
return head ;

}

And the swap function:

 struct node *swap_with_next(struct node *n) {
      struct node *tmp ;
      tmp = n ;
      n = n->next ;
      tmp->next = n->next ;
      n->next = tmp ;
      return n ;
 }

Problem: Incorrect output:

input:  5->2->NULL
output: 5->NULL

input:  9->1->5->2->8->3
output: 9->5->8

Any help would be greatly appreciated!

Answer 1

Ok, so, let’s say there are 3 nodes A, B and C (in that order). You are trying to swap B and C. So, you are passing the pointer to B to the swap function. Now, all the pointer manipulations are correct but, you miss one thing. A’s next pointer should point to C when you finish the swap. You never set this. When you finish the swap function, A’s next pointer still points to B which is obviously not correct. That’s the problem.