21、Merge Two Sorted Lists

 1     ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
 2         ListNode* t;
 3         t=(ListNode*)malloc(sizeof(ListNode));
 4         ListNode *t1;
 5         t1=t;
 6         if(l1==NULL&&l2==NULL)
 7             return NULL;
 8         while(l1&&l2)
 9         {
10             if(l1->val<=l2->val)
11             {
12                 t1->next=l1;
13                 l1=l1->next;
14             }
15             else
16             {
17                 t1->next=l2;
18                 l2=l2->next;
19             }
20             t1=t1->next;
21         }
22         if(l1==NULL)
23         {
24             t1->next=l2;
25         }
26         if(l2==NULL)
27         {
28             t1->next=l1;
29         }
30         return t->next;