#include <stdio.h>
#include <stdlib.h>

struct NodeStruct {
	int value;
	struct NodeStruct *next;
};

typedef struct NodeStruct Node;

struct ListStruct {
	int length;
	Node *head;
	Node *tail;
};

typedef struct ListStruct List;

void split(List *l, List *half1, List *half2)
{
	int i;
	Node *iter;

	iter = l->head;
	for (i = 0; i < l->length / 2 - 1; ++i) {
		iter = iter->next;
	}
	half1->head = l->head;
	half1->tail = iter;
	half1->length = l->length / 2;
	half2->head = iter->next;
	half2->tail = l->tail;
	half2->length = l->length - half1->length;
	half1->tail->next = NULL;
}

void merge(List *list1, List *list2, List *result)
{
	Node *iter1;
	Node *iter2;

	iter1 = list1->head;
	iter2 = list2->head;

	if (iter1->value <= iter2->value) {
		result->head = iter1;
		result->tail = iter1;
		iter1 = iter1->next;
	} else {
		result->head = iter2;
		result->tail = iter2;
		iter2 = iter2->next;
	}
	while (iter1 && iter2) {
		if (iter1->value <= iter2->value) {
			result->tail->next = iter1;
			result->tail = iter1;
			iter1 = iter1->next;
		} else {
			result->tail->next = iter2;
			result->tail = iter2;
			iter2 = iter2->next;
		}
	}
	if (iter1) {
		result->tail->next = iter1;
		result->tail = list1->tail;
	} else if (iter2) {
		result->tail->next = iter2;
		result->tail = list2->tail;
	}
	result->length = list1->length + list2->length;
}

void mergesort(List *l)
{
	List half1;
	List half2;

	if (l->length <= 1)
		return;

	split(l, &half1, &half2);
	mergesort(&half1);
	mergesort(&half2);
	merge(&half1, &half2, l);
}

gruß danke