LeetCode OJ：Merge Two Sorted Lists（合并两个链表）

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.

注意题目要求合并的时候不能新建节点，直接使用原来的节点，比较简单，代码如下：

 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 * root = new ListNode(-1);
15         ListNode * helper = root;
16         while(l1!=NULL && l2!=NULL){
17             if(l1->val <= l2->val)
18                 root->next = l1, l1=l1->next;
19             else if(l1->val > l2->val)
20                 root->next = l2, l2=l2->next;
21             root = root->next;
22         }
23         while(l1!=NULL){
24             root->next = l1;
25             l1 = l1->next;
26             root = root->next;
27         }
28         while(l2!=NULL){
29             root->next = l2;
30             l2 = l2->next;
31             root = root->next;
32         }
33         return helper->next;
34     }
35 };