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.

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 dummy(0);
        
        ListNode *tail = &dummy;
        ListNode *cur = NULL;
        
        while(l1 != NULL && l2 != NULL)
        {
            if(l1->val >= l2->val)
            {
                cur = l2;
                l2 = l2->next;
            }
            else
            {
                cur = l1;
                l1 = l1->next;
            }
            tail->next = cur;
            tail = cur;
        }
        if(l1 != NULL)
            tail->next = l1;
        if(l2 != NULL)
            tail->next = l2;
        
        return dummy.next;
    }
};