Leetcode: 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.

这道题和前面Leetcode: Merge Sorted Array 这道题看似类似，因为数据结构的不同，但是解法还是有一些不同的。

C++代码示例：

/** * 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)    {        //helper指向l1和l2指向最小的哪一个，一直循环着找下去        ListNode *helper = new ListNode(0);        ListNode *head = helper;        while (l1 && l2)        {            if (l1->val < l2->val)            {                helper->next = l1;                l1 = l1->next;            }            else            {                helper->next = l2;                l2 = l2->next;            }            helper = helper->next;        }        //如果最后l1或者l2还有指向的数据，直接接到helper的后面        if (l1) helper->next = l1;        if (l2) helper->next = l2;        return head->next;    }};