A “deque” is a data structure consisting of a list of items, on which the following operations are possible:

Push(X,D): Insert item X on the front end of deque D.
Pop(D): Remove the front item from deque D and return it.
Inject(X,D): Insert item X on the rear end of deque D.
Eject(D): Remove the rear item from deque D and return it. Write routines to support the deque that take O(1) time per operation.
Format of functions:
Deque CreateDeque();
int Push( ElementType X, Deque D );
ElementType Pop( Deque D );
int Inject( ElementType X, Deque D );
ElementType Eject( Deque D );

where Deque is defined as the following:

typedef struct Node *PtrToNode;
struct Node {
    
    
    ElementType Element;
    PtrToNode Next, Last;
};
typedef struct DequeRecord *Deque;
struct DequeRecord {
    
    
    PtrToNode Front, Rear;
};

Here the deque is implemented by a doubly linked list with a header. Front and Rear point to the two ends of the deque respectively. Front always points to the header. The deque is empty when Front and Rear both point to the same dummy header. Note: Push and Inject are supposed to return 1 if the operations can be done successfully, or 0 if fail. If the deque is empty, Pop and Eject must return ERROR which is defined by the judge program.

Sample program of judge:

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

#define ElementType int
#define ERROR 1e5
typedef enum {
    
     push, pop, inject, eject, end } Operation;

typedef struct Node *PtrToNode;
struct Node {
    
    
    ElementType Element;
    PtrToNode Next, Last;
};
typedef struct DequeRecord *Deque;
struct DequeRecord {
    
    
    PtrToNode Front, Rear;
};
Deque CreateDeque();
int Push( ElementType X, Deque D );
ElementType Pop( Deque D );
int Inject( ElementType X, Deque D );
ElementType Eject( Deque D );

Operation GetOp();          /* details omitted */
void PrintDeque( Deque D ); /* details omitted */

int main()
{
    
    
    ElementType X;
    Deque D;
    int done = 0;

    D = CreateDeque();
    while (!done) {
    
    
        switch(GetOp()) {
    
    
        case push: 
            scanf("%d", &X);
            if (!Push(X, D)) printf("Memory is Full!\n");
            break;
        case pop:
            X = Pop(D);
            if ( X==ERROR ) printf("Deque is Empty!\n");
            break;
        case inject: 
            scanf("%d", &X);
            if (!Inject(X, D)) printf("Memory is Full!\n");
            break;
        case eject:
            X = Eject(D);
            if ( X==ERROR ) printf("Deque is Empty!\n");
            break;
        case end:
            PrintDeque(D);
            done = 1;
            break;
        }
    }
    return 0;
}

/* Your function will be put here */

Sample Input:

Pop
Inject 1
Pop
Eject
Push 1
Push 2
Eject
Inject 3
End

Sample Output:

Deque is Empty!
Deque is Empty!
Inside Deque: 2 3

见代码注释

//Last相当于Pre的意思，因为是循环链表，所以前一个能找到下一个
//下一个能找到前一个 ,Last有上一个，过去的意思 

Deque CreateDeque(){
    
    
    Deque de = (Deque)malloc(sizeof(struct DequeRecord));
    PtrToNode ptr = (PtrToNode)malloc(sizeof(struct Node));
    ptr->Next = NULL;
    //不写也可以，因为再插入时会对其指向作出确定，若写则置为空
    ptr->Last = NULL;
    de->Front = de->Rear = ptr;
    return de;
}

int Push(ElementType X, Deque D){
    
    
	//创建新的结点 
    PtrToNode ptr = (PtrToNode)malloc(sizeof(struct Node));
    ptr->Element = X;
    //链表为空 
    if (D->Front==D->Rear){
    
    
        ptr->Next = NULL;
        ptr->Last = D->Front;
        D->Front->Next = ptr;
        D->Rear = ptr;
    }
    //至少有一个结点 头插法 
    else{
    
    
        ptr->Next = D->Front->Next;
        ptr->Last = D->Front;
        D->Front->Next = ptr;
        ptr->Next->Last = ptr;
    }
    return 1;
}

ElementType Pop(Deque D){
    
    
	//空 
    if (D->Front==D->Rear)
        return ERROR;
    PtrToNode ptr = D->Front->Next;//指向第一个数据结点 
    int x = ptr->Element;
    if (ptr==D->Rear){
    
    
        D->Front->Next = NULL;
        D->Rear = D->Front;
        ptr->Last = NULL;//不写也可以，若写置为空
    }
    else{
    
    
        D->Front->Next = ptr->Next;
        ptr->Next->Last = D->Front;
    }
    free(ptr);
    return x;
}

int Inject(ElementType X, Deque D){
    
    
    PtrToNode ptr = (PtrToNode)malloc(sizeof(struct Node));
    ptr->Element = X;
    ptr->Next = NULL;
    ptr->Last = D->Rear;
    D->Rear->Next = ptr;
    D->Rear = ptr;
    return 1;
}

ElementType Eject(Deque D){
    
    
	//空 
    if (D->Front==D->Rear)
        return ERROR;
    PtrToNode ptr = D->Rear;
    int x = ptr->Element;
    ptr->Last->Next = NULL;
    D->Rear = ptr->Last;
    free(ptr);
    return x;
}