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.

/** * 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 dummy(-1);        ListNode* p = &dummy;        if (l1->val < l2->val)        {            p->next = l1;            p = p->next;            p->next = mergeTwoLists(p->next,l2);        }        else        {            p->next = l2;            p = p->next;            p->next =mergeTwoLists(l1,p->next);        }        return dummy.next;    }};