归并
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* merge(ListNode* l1, ListNode* l2) {
ListNode * dummy=new ListNode(-1);//新建1个虚头节点 相当于malloc
ListNode * tail=dummy;
while(l1!=NULL&&l2!=NULL)
{
if(l1->val<l2->val)
{
tail->next=l1;
l1=l1->next;
tail=tail->next;
}
else
{
tail->next=l2;
l2=l2->next;
tail=tail->next;
}
}
if(l1!=NULL)
{
tail->next=l1;
}
else
{
tail->next=l2;
}
return dummy->next;
}
};