/**
 * 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) {
        ListNode* L = new ListNode(-1); //意思是调用单链表结构体的构造函数创建了一个头结点为-1的单链表
        ListNode* r = L;
        ListNode* p = l1, * q = l2; //p指针指向l1,q指针指向l2
        while(p && q){
            if(p->val <= q->val){
                r->next = p;
                r = p;
                p = p->next;
            }else{
                r->next = q;
                r = q;
                q = q->next;
            }
        }
        if(!p) r->next = q;
        if(!q) r->next = p;

        return L->next; //返回头结点的下一结点,从首元结点开始输出
    }
};