Sort a linked list in O(n log n) time using constant space complexity.

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

typedef struct ListNode{
    int val;
    struct ListNode *next;
}ListNode;

ListNode *mergesort(ListNode *head1, ListNode *head2) {
    ListNode *q, *p, *tmp, *head;
    head = (ListNode*)malloc(sizeof(ListNode));
    tmp = head;
    for(p = head1, q = head2; p != NULL && q != NULL; ) {
        if(p->val > q->val) {
            tmp->next = q;
            q = q->next;
        }
        else {
            tmp->next = p;
            p = p->next;
        }
        tmp = tmp->next;
    }
    if(p != NULL) tmp->next = p;
    else if(q != NULL) tmp->next = q;
    return head->next;
}

ListNode *sortList1(ListNode *first, ListNode *last) {
    ListNode *p1, *mid, *midnext;
    ListNode *head1, *head2;
    if(first == NULL || first->next == NULL) return first;
    for(p1 = first, mid = first; p1 != last && p1->next != last; ) {
        mid = mid->next;
        p1 = p1->next->next;
    }
    midnext = mid->next;
    mid->next = NULL;
    //printf("head1:%d, head2:%d\n", first->val, midnext->val);
    head1 = sortList1(first, mid);
    head2 = sortList1(midnext, last);
    return mergesort(head1, head2);
}

ListNode *sortList(ListNode *head) {
    ListNode *last = head;
    if(head == NULL || head->next == NULL) return head;
    for( ; last->next != NULL; last = last->next) ; 
    //printf("head:%d, last:%d\n", head->val, last->val);
    return sortList1(head, last);
}

void main() {
    ListNode *head, *p1, *p2;
    int i = 0;
    p1 = p2 = (ListNode *)malloc(sizeof(ListNode));
    scanf("%d", &p1->val);
    while(p1->val != 0) {
        i++;
        if(i == 1) head = p1;
        else p2->next = p1;
        p2 = p1;
        p1 = (ListNode *)malloc(sizeof(ListNode));
        scanf("%d", &p1->val);
    }
    p2->next = NULL;
    for(p1 = head; p1 != NULL; p1 = p1->next) printf("%d ", p1->val);
    printf("\n");
    head = sortList(head);
    for(p1 = head; p1 != NULL; p1 = p1->next) printf("%d ", p1->val);
    printf("\n");
}

Sort List