LeetCode 21 Merge Two Sorted Lists 合并两个有序链表

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.
Example:
Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if(!l1&&!l2)
            return nullptr;
        if(!l1)
            return l2;
        if(!l2)
            return l1;
        ListNode* first=new ListNode(-1);
        ListNode* cur=first;
        while(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?l1:l2;
        return first->next;
    }
};