C语言链队列的数据结构问题

Question

#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
typedef struct node
{
    int data;
    struct node *next;
}Node;
typedef struct
{
    Node *front;
    Node *rear;
}lqueuetp;

void InitQueue(lqueuetp *lq)
{
    //设置一个空的链队列lq
    lq->front = (Node*)malloc(sizeof(Node));
    lq->front->next = NULL;
    lq->front = lq->rear;        //链队列空的判断条件 
}

void QueueEmpty(lqueuetp *lq)
{
    if(lq->front = lq->rear)
        printf("链队列为空");
    else
        printf("链队列不为空");    
}

int Head(lqueuetp *lq)        //读链队列队头元素 
{
    //若链队列lq不为空，则返回队头元素值，否则返回0
    if(lq->front == lq->rear)
        return 0;
    else
        return(lq->front->next->data);
}

void EnQueue(lqueuetp *lq, int x)
{
    //在链队列lq中，插入x为新的队尾元素
    Node *p;
    p = (Node*)malloc(sizeof(Node));
    p->data = x;
    p->next = NULL;
    lq->rear->next = p;        //尾插 
    lq->rear = p; 
}

int DelQueue(lqueuetp *lq)
{
    //若链队列lq不为空，则删去队头元素并返回元素值，否则返回0
    int x;
    Node *q;
    q = (Node*)malloc(sizeof(Node));
    if(lq->front == lq->rear)
        return 0;
    else
    {
        q = lq->front->next;
        lq->front->next = q->next;
        if(q->next == NULL)    
            lq->rear = lq->front;            //当链队列中仅有一个结点时，出队时还需修改尾指针
        x = q->data;
        free(q);
        return x; 
    } 
}

int main()
{
    int x,i,n;
    printf("链队列长度：");
    scanf("%d",&n);
    lqueuetp *lq;
    lq = (lqueuetp*)malloc(sizeof(lqueuetp));
    InitQueue(lq);
    printf("入队列：\n");
    for(i = 1; i <= n;i++)
    {
        scanf("%d",&x);
        EnQueue(lq,x);
    }
    QueueEmpty(lq);
    printf("读取队列对头元素：%d\n",Head(lq));
    printf("出队列：%d\n",DelQueue(lq));
    return 0;
}

这个代码哪里出现问题了？为什么入队列时不成功？

Answer 1

修改如下，改动处见注释，供参考：

#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
typedef struct node
{
    int    data;
    struct node *next;
}Node;
typedef struct
{
    Node *front;//队头指针
    Node *rear; //队尾指针
}lqueuetp;

void InitQueue(lqueuetp *lq)
{
    //设置一个空的链队列lq
    //lq->front = (Node*)malloc(sizeof(Node));  修改
    //lq->front->next = NULL;                   修改
    lq->front = lq->rear = NULL;  //链队列空的判断条件
}

void QueueEmpty(lqueuetp *lq)
{
    if(!lq->front)  //if(lq->front = lq->rear) 修改
        printf("链队列为空\n");
    else
        printf("链队列不为空\n");
}

int Head(lqueuetp *lq)        //读链队列队头元素 
{
    //若链队列lq不为空，则返回队头元素值，否则返回0
    if(!lq->front) //if(lq->front == lq->rear)         修改
        return 0;
    else
        return lq->front->data ; //(lq->front->next->data);修改
}

void EnQueue(lqueuetp *lq, int x)
{
    //在链队列lq中，插入x为新的队尾元素
    Node *p;
    p = (Node*)malloc(sizeof(Node));
    p->data = x;
    p->next = NULL;

    if (!lq->front)     //修改
        lq->front = p;  //修改
    else                //修改
        lq->rear->next = p;//尾插
    lq->rear = p;       //修改
}

int DelQueue(lqueuetp *lq)
{
    //若链队列lq不为空，则删去队头元素并返回元素值，否则返回0
    int x;
         //Node *q;                        //修改
         //q = (Node*)malloc(sizeof(Node));//修改
    if(!lq->front)  //if(lq->front == lq->rear)//修改
        return 0;
    //else                                //修改
    //{                                   //修改
    x = lq->front->data;
        //q = lq->front->next;            //修改
        //lq->front->next = q->next;      //修改
    if(lq->front->next == NULL){    //只剩一个结点
        free(lq->front);
        lq->front = lq->rear = NULL;
    }
    else{                          //多个结点
        Node *q = lq->front->next;
        free(lq->front);
        lq->front = q;
    }
        //lq->rear = lq->front;            //当链队列中仅有一个结点时，出队时还需修改尾指针
        //x = q->data;
        //free(q);
    return x;
    //}
}

int main()
{
    int x,i,n;
    printf("链队列长度：");
    scanf("%d",&n);
    lqueuetp *lq;
    lq = (lqueuetp*)malloc(sizeof(lqueuetp));
    InitQueue(lq);
    printf("入队列：\n");
    for(i = 1; i <= n;i++)
    {
        scanf("%d",&x);
        EnQueue(lq,x);
    }
    QueueEmpty(lq);

    printf("读取队列对头元素：%d\n",Head(lq));

    x = DelQueue(lq);                        //修改
    printf(x ? "出队列：%d\n" : "队空\n",x); //修改
    x = DelQueue(lq);                        //修改
    printf(x ? "出队列：%d\n" : "队空\n",x); //修改

    return 0;
}