合并两个有序单向链表。
一次AC
附上代码：

class Solution {
public:
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        // Note: The Solution object is instantiated only once and is reused by each test case. 
        if(l1 == NULL)
            return l2;
        if(l2 == NULL)
            return l1;
        int val;
        if(l1->val < l2->val)
        {
            val = l1->val;
            l1 = l1->next;
        }
        else
        {
            val = l2->val;
            l2 = l2 ->next;
        }
        ListNode * l = new ListNode(val);
        ListNode * lcopy = l;
        int flag = 0;
        while(true)
        {
            if(l1 == NULL)
            {
                flag = 1;
                break;
            }
            if(l2 == NULL)
            {
                flag = 2;
                break;
            }
            if(l1->val < l2->val)
            {
                val = l1->val;
                l1 = l1->next;
            }
            else
            {
                val = l2->val;
                l2 = l2 ->next;
            }
            ListNode * tmp = new ListNode(val);
            lcopy->next = tmp;
            lcopy = lcopy->next;
        }
        if(flag == 1)
        {
            while(l2 != NULL)
            {
                val = l2->val;
                l2 = l2->next;
                ListNode * tmp = new ListNode(val);
                lcopy->next = tmp;
                lcopy = lcopy->next;
            }
        }
        if(flag == 2)
        {
            while(l1 != NULL)
            {
                val = l1->val;
                l1 = l1->next;
                ListNode * tmp = new ListNode(val);
                lcopy->next = tmp;
                lcopy = lcopy->next;
            }
        }
        return l;
    }
};