把奇数位的数依次放入一个链表，偶数位的数放入另外一个链表，
然后奇数位链表的末尾指向偶数位的链表，同时把偶数位的链表末尾设置为NULL（容易忘记，切记！！！）
struct ListNode* oddEvenList(struct ListNode* head) {
struct ListNode *odd = calloc(1, sizeof(struct ListNode));
struct ListNode *even = calloc(1, sizeof(struct ListNode));
struct ListNode *lastodd, *lasteven;
lastodd = odd;
lasteven = even;
int oddtag = 1;
struct ListNode * p = head;
while(p){
if(oddtag){
lastodd->next = p;
lastodd = lastodd->next;
oddtag = 0;
}else{
lasteven->next = p;
lasteven = lasteven->next;
oddtag = 1;
}
p = p->next;
}
lastodd->next = even->next;
lasteven->next = NULL;
free(odd);
free(even);
return head;
}