[Leetcode]Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

一直没想出原址排序的办法，只能额外开辟空间了。

class Solution {
public:
ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
ListNode *helper=new ListNode(0);
ListNode *head=helper;
while(l1 && l2)
{
if(l1->val<l2->val) helper->next=l1,l1=l1->next;
else helper->next=l2,l2=l2->next;
helper=helper->next;
}
if(l1) helper->next=l1;
if(l2) helper->next=l2;
return head->next;
}
};