/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* merge(ListNode* l1, ListNode* l2) {
        //归并的思路
        auto dummy = new ListNode(-1);
        auto cur = dummy;
        while(l1 && l2){    //注意不是!l1 && !l2
            if(l1->val < l2->val)
            {
                cur->next = l1;
                l1 = l1->next;
            }
            else
            {
                cur->next = l2;
                l2 = l2->next;
            }
            cur = cur-> next;
        }
        cur->next = (l1 != nullptr ? l1 : l2);
        return dummy->next;
    }  
};