力扣-21題 合并兩個有序連結清單（C++）- 連結清單向後添加節點

題目連結：https://leetcode-cn.com/problems/merge-two-sorted-lists/

題目如下：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode* head=new ListNode(-1);
        ListNode* p=head;

        while(l1&&l2){
            if(l1->val < l2->val){
                p->next=l1;
                l1=l1->next;
            }
            else{
                p->next=l2;
                l2=l2->next;
            }
            p=p->next;
        }

        if(l1) p->next=l1;
        else p->next=l2;

        return head->next;

    }
};