【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.

 struct ListNode {
     int val;
     ListNode *next;
     ListNode(int x) : val(x), next(NULL) {}

 };
 ListNode *mergeTwoLists(ListNode *l1, ListNode *l2)
 {
     if (l2 == NULL)
         return l1;
     if (l1 == NULL)
         return l2;
     ListNode  *r, *head;
     head = NULL;
     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
     {
         head = l1;
         l1 = l1->next;
     }
     else
     {
         head = l2;
         l2 = l2->next;
     }
     r = head;
     while (l1&&l2)    //比较大小
     {
         if (l1->val <= l2->val)
         {
             r->next = l1;
             l1 = l1->next;
         }
         else
         {
             r->next = l2;
             l2 = l2->next;
         }
         r = r->next;
     }
     if (l1 == NULL)   //l1为空，直接连接l2
     {
         r->next = l2;
     }
     if (l2 == NULL)  //l2为空，直接连接l1
     {
         r->next = l1;
     }
     return head;
 }