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：

 /**
  * Definition for singly-linked list.
  * struct ListNode {
  *     int val;
  *     ListNode *next;
  *     ListNode(int x) : val(x), next(NULL) {}
  * };
  */
 class Solution {
 public:
     ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
         if(l1 == NULL) return l2;
         if(l2 == NULL) return l1;
         ListNode * root = new ListNode(-);
         ListNode * helper = root;
         while(l1!=NULL && l2!=NULL){
             if(l1->val <= l2->val)
                 root->next = l1, l1=l1->next;
             else if(l1->val > l2->val)
                 root->next = l2, l2=l2->next;
             root = root->next;
         }
         while(l1!=NULL){
             root->next = l1;
             l1 = l1->next;
             root = root->next;
         }
         while(l2!=NULL){
             root->next = l2;
             l2 = l2->next;
             root = root->next;
         }
         return helper->next;
     }
 };

 public class Solution {
     public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
         ListNode helper = new ListNode(0);
         ListNode ret = helper;
         if(l1 == null) return l2;
         if(l2 == null) return l1;
         while(l1 != null && l2 != null){
             if(l1.val < l2.val){
                 helper.next = l1;
                 l1 = l1.next;
             }else{
                 helper.next = l2;
                 l2 = l2.next;
             }
             helper = helper.next;
         }
         while(l1 != null){
             helper.next = l1;
             l1 = l1.next;
             helper = helper.next;
         }
         while(l2 != null){
             helper.next = l2;
             l2 = l2.next;
             helper = helper.next;
         }
         return ret.next;
     }
 }