申请一个dummy头，依次从头比较两个链表，谁小就插入到 dummy，对应指针后移
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* struct ListNode *next;
* };
*/
struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2) {
struct ListNode *dummyhead = malloc(sizeof(struct ListNode));
dummyhead->next = NULL;
struct ListNode *lastnode = dummyhead;
while(1){
if(l1&&l2){
if(l1->val < l2->val){
lastnode->next = l1;
l1 = l1->next;
lastnode = lastnode->next;
}else{
lastnode->next = l2;
l2 = l2->next;
lastnode = lastnode->next;
}
}else if(l1){
lastnode->next = l1;
break;
}else if(l2){
lastnode->next = l2;
break;
}else
break;
}
struct ListNode *tmp = dummyhead->next;
free(dummyhead);
return tmp;
}