【LeetCode OJ】Merge Two Sorted Lists

题目：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.

 1 struct ListNode {

 2     int val;

 3     ListNode *next;

 4     ListNode(int x) : val(x), next(NULL) {}

 5     

 6 };

 7 ListNode *mergeTwoLists(ListNode *l1, ListNode *l2)

 8 {

 9     if (l2 == NULL)

10         return l1;

11     if (l1 == NULL)

12         return l2;

13     ListNode  *r, *head;

14     head = NULL;

15     if (l1->val <= l2->val)  //哪个链表的第一个节点小就把头指针指向哪个节点

16     {

17         head = l1;

18         l1 = l1->next;

19     }

20     else

21     {

22         head = l2;

23         l2 = l2->next;

24     }

25     r = head;

26     while (l1&&l2)    //比较大小

27     {

28         if (l1->val <= l2->val)

29         {

30             r->next = l1;

31             l1 = l1->next;

32         }

33         else

34         {

35             r->next = l2;

36             l2 = l2->next;

37         }

38         r = r->next;

39     }

40     if (l1 == NULL)   //l1为空，直接连接l2

41     {

42         r->next = l2;

43     }

44     if (l2 == NULL)  //l2为空，直接连接l1

45     {

46         r->next = l1;

47     }

48     return head;

49 }