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.

不申请新的节点（把两个链表的节点接起来）

提交通过：

 ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode v(0),*pre = &v,*p = pre;  // pre带头结点
        while(l1 != NULL && l2 != NULL){
            if(l1->val > l2 ->val){
                p->next = l2;
                l2 = l2->next;
                p = p->next;
            }
            else{
                p->next = l1;
                l1 = l1->next;
                p = p->next;
            }
        }
        if(l1 == NULL ) p->next = l2;
        else p->next = l1;
        return pre->next;
    }