21合并两个有序链表

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
12         if(l1==NULL) return l2;
13         if(l2==NULL) return l1;
14         ListNode *phead,*p;//定义头节点
15         phead=new ListNode(0);
16         phead->next=NULL;
17         p=phead;
18         while(l1!=NULL&&l2!=NULL){
19             if(l1->val<=l2->val){
20                // l1->next=p->next;
21                 p->next=l1;
22                 l1=l1->next;
23             }
24             else{
25                // l2->next=p->next;
26                 p->next=l2;
27                 l2=l2->next;
28             }
29             p=p->next;
30         }
31         p->next=l1!=NULL?l1:l2;//判断哪一个先为空
32         return phead->next;
33     }
34 };