双向链表的遍历和生成

双向链表，又称为双链表，是链表的一种，它的每个数据结点都有两个指针，分别指向直接后继和直接前驱。所以，双向链表中的任意一个结点开始，都可以很方便地访问它的前驱结点和后继结点。

基本操作：

#include 
using namespace std;
class Node {
   public:
    int dat;
    Node *pre;
    Node *next;
    Node(int val) {
        dat = val;
        pre = next = NULL;
    }
};
class TList {
   private:
    Node *first;
    Node *end;

   public:
    TList() { first = end = NULL; }
    void append(int val);
    void travalList() const;
    void retravalList() const;

    ~TList() {
        Node *p;
        Node *t;
        p = this->first;
        while (p) {
            t = p;
            p = p->next;
            delete t;
        }
        first = end = NULL;
    }
};
// StudybarCommentEnd
void TList::append(int val) {
    Node *temp = new Node(val);
    if (this->first == NULL) {
        this->first = temp;
        this->end = temp;
    } else if (this->first == this->end) {
        this->first->next = temp;
        temp->pre = this->first;
        this->end = temp;
    } else {
        this->end->next = temp;
        temp->pre = this->end;
        this->end = temp;
    }
}

void TList::travalList() const {
    Node *temp = this->first;
    while (temp) {
        cout << temp->dat << " ";
        temp = temp->next;
    }
    cout << endl;
}

void TList::retravalList() const {
    Node *temp = this->end;
    while (temp) {
        cout << temp->dat << " ";
        temp = temp->pre;
    }
    cout << endl;
}

int main(void) {
    int a, b, c, d;
    cin >> a >> b >> c >> d;
    TList t;
    t.append(a);
    t.append(b);
    t.append(c);
    t.append(d);
    t.travalList();
    t.retravalList();
    return 1;
}