【21】Merge Two Sorted Lists

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.

简单的链表题，加一个头节点方便编码

ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {    ListNode* head=new ListNode(0);    ListNode* p=head;    while(l1 || l2){        if((l1 && l2 && (l1->val < l2->val)) || !l2){            p->next=l1;            l1=l1->next;            p=p->next;        }        else{            p->next=l2;            l2=l2->next;            p=p->next;        }    }    return head->next;}