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

ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if (l1&&!l2||!l1&&!l2) return l1;  //输入判断
        if (!l1&&l2) return l2;
        
        ListNode *new_head,*tail;// tail指向新链表的表尾
        if (l1->val<l2->val)
        {
            new_head=l1;
            l1=l1->next;
        }
        else 
        {
            new_head=l2;
            l2=l2->next;
        }
        tail=new_head;
        while(l1||l2)
        {
            if (l1&&l2)
            {
                if(l1->val<=l2->val)
                {
                    tail->next=l1;
                    l1=l1->next;
                    tail=tail->next;
                }
                else
                {
                    tail->next=l2;
                    l2=l2->next;
                    tail=tail->next;
                }
            }
            else if(l1)
            {
                tail->next = l1;
                return new_head;
            }
            else 
            {
                tail->next = l2;
                return new_head;
            }
        }
        
        return new_head;
    }