/**
 * 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) {
        auto head = new ListNode(-1),last = head;
        while(l1 && l2){  // 两个都还存在才能比较
            if(l1->val < l2->val){
                last = last->next = l1;
                //本来是：l1->next = l1->next->next; 但其实可以消掉
                l1 = l1->next;
            }
            else{
                last = last->next = l2;
                l2 = l2->next;
            }
        }

        // 比较到最后最终会剩下一个（两链表中最大的）
        if(l1)last = last->next = l1;
        if(l2)last = last->next = l2;

        return head->next;
    }
};