思路：1、反转后头结点变化；2、注意链表可能断裂的情形

#include <iostream>
using namespace std;
struct Node {
    int data;
    Node* next;
};
class List{
private:
    int N;
    Node* first;
public:
    int size() { return N; }
    bool isEmpty() { first==NULL; }
    void insert(int val){
        Node* oldfirst=first;
        first=new Node();
        first->data=http://www.mamicode.com/val;>