双链表

双链表相对于单链表来说，多了只想前一个Node的指针。假如有N个node的话，多了N个指针，换来了比单链更灵活的遍历（可以倒着来了），是不是需要这样，it all depends.

双链表实验：one way or round trip? round trip!

#include <stdio.h>
#include <stdlib.h>

struct DList{
    int data;
    DList* prev;
    DList* next;
    DList(){}
    DList(int d){
        data = d;
        prev = NULL;
        next = NULL;
    }
    DList(int d, DList* p, DList* n){
        data = d;
        prev = p;
        next = n;
    }
};

DList* appendToTail(int d, DList* head){
    DList* p = head;
    while(p->next!=NULL){
        p = p->next;
    }
    DList* temp = new DList(d,p,NULL);
    p->next = temp;
    return head;
}

void print_round_trip(DList* head){
    DList* p = head;
    DList* temp;
    while(p!=NULL){
        if(p->next==NULL){
            printf("%d\n",p->data);
        }else{
            printf("%d -> ",p->data);
        }
        temp = p;
        p = p->next;
    }

    //the last node;
    p = temp;
    while(p!=NULL){
        if(p->prev==NULL){
            printf("%d\n",p->data);
        }else{
            printf("%d -> ",p->data);
        }
        p = p->prev;
    }    
}
int main(){
    DList* head = new DList(23);
    head = appendToTail(21,head);
    head = appendToTail(2,head);
    head = appendToTail(877,head);
    print_round_trip(head);
    return 0;
}