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.

Example:

Input: 1->2->4, 1->3->4Output: 1->1->2->3->4->4

不带头结点

/** * 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 *first = NULL;        if(l1->val < l2->val) {            first = l1;            l1 = l1->next;        } else {            first = l2;            l2 = l2->next;        }        ListNode *last = first;        while(l1 && l2) {            if(l1->val < l2->val) {                last->next = l1;                l1 = l1->next;            } else {                last->next = l2;                l2 = l2->next;            }            last = last->next;        }        if(l1)            last->next = l1;        if(l2)            last->next = l2;        return first;            }};