complexity of mergesort with linked list

Question

i have code for mergesort using linked list,it works fine,my question what is complexity of this algorithm?is it O(nlog(n))?also is it stable?i am interested because as i

Answer 1

Mergesort means split&merge. The splitting in the fragment below is not perfect (it leads to quadratic behavior on equally distributed runlengths, see the comment from Christoph)), but it will do the trick:

#include 
#include 

struct llist {
        struct llist *next;
        char *payload;
        };

int llist_cmp(struct llist *l, struct llist *r);
struct llist * llist_split(struct llist **hnd
                        , int (*cmp)(struct llist *l, struct llist *r) );
struct llist * llist_merge(struct llist *one, struct llist *two
                        , int (*cmp)(struct llist *l, struct llist *r) );
struct llist * llist_sort(struct llist *ptr
                        , int (*cmp)(struct llist *l, struct llist *r) );

struct llist * llist_split(struct llist **hnd, int (*cmp)(struct llist *l, struct llist *r) )
{
struct llist *this, *save, **tail;

for (save=NULL, tail = &save; this = *hnd; ) {
        if (! this->next) break;
        if ( cmp( this, this->next) <= 0) { hnd = &this->next; continue; }
        *tail = this->next;
        this->next = this->next->next;
        tail = &(*tail)->next;
        *tail = NULL;
        }
return save;
}

struct llist * llist_merge(struct llist *one, struct llist *two, int (*cmp)(struct llist *l, struct llist *r) )
{
struct llist *result, **tail;

for (result=NULL, tail = &result; one && two; tail = &(*tail)->next ) {
        if (cmp(one,two) <=0) { *tail = one; one=one->next; }
        else { *tail = two; two=two->next; }
        }
*tail = one ? one: two;
return result;
}
struct llist * llist_sort(struct llist *ptr, int (*cmp)(struct llist *l, struct llist *r) )
{
struct llist *save;

save=llist_split(&ptr, cmp);
if (!save) return ptr;

save = llist_sort(save, cmp);

return llist_merge(ptr, save, cmp);
}

int llist_cmp(struct llist *l, struct llist *r)

{
if (!l) return 1;
if (!r) return -1;
return strcmp(l->payload,r->payload);
}


struct llist lists[] =
{{ lists+1, "one" }
,{ lists+2, "two" }
,{ lists+3, "three" }
,{ lists+4, "four" }
,{ lists+5, "five" }
,{ lists+6, "six" }
,{ lists+7, "seven" }
,{ lists+8, "eight" }
,{ lists+9, "nine" }
,{ NULL, "ten" }
        };

int main()
{
struct llist *root,*tmp;

root = lists;

fprintf(stdout, "## %s\n", "initial:" );
for (tmp=root; tmp; tmp=tmp->next) {
        fprintf(stdout, "%s\n", tmp->payload);
        }

fprintf(stdout, "## %s\n", "sorting..." );
root = llist_sort(root, llist_cmp);
for (tmp=root; tmp; tmp=tmp->next) {
        fprintf(stdout, "%s\n", tmp->payload);
        }
fprintf(stdout, "## %s\n", "done." );

return 0;
}