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.

题目：合并两个排序后的单链表，要求合并之后也是排序好的

思路：递归合并

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     struct ListNode *next;  * }; */struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2) {    if(NULL == l1) return l2;    if(NULL == l2) return l1;    struct ListNode* MergeList = NULL;    if(l1->val < l2->val)    {        MergeList = l1;        MergeList->next = mergeTwoLists(l1->next,l2);    }    else    {        MergeList = l2;        MergeList->next = mergeTwoLists(l1,l2->next);    }    return MergeList;}