21 Merge Two Sorted Lists(两链表归并排序Easy)

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
ListNode* head=new ListNode(0);
ListNode* p=head;
while(l1&&l2){
if(l1->val<=l2->val){
p->next=l1;
l1=l1->next;
p=p->next;
}
else{
p->next=l2;
l2=l2->next;
p=p->next;
}
}
if(l1)p->next=l1;
if(l2)p->next=l2;
return head->next;    //第一个节点去掉
}
};