一、关键点

1. 队头队尾

队头是front,队尾是rear；
队头出数据，队尾进数据；
队头指针front 不存数据；
当front == rear时 队列为空；
清空或取出数据至队列为空时记得将rear指针移到front上；

  QueuePtr p = Q->front->next;
    *e = p->data;
    Q->front->next = p->next;
    if(p==Q->rear)
    {
        Q->rear = Q->front;
    }

2.取出数据要free()结点

 QueuePtr p = Q->front->next;
    *e = p->data;
    Q->front->next = p->next;
    if(p==Q->rear)
    {
        Q->rear = Q->front;
    }
    free(p);

二、代码

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

#define STACK_INIT_SIZE 100
#define STACK_INCREMENT 10
#define OVERFLOW -2
#define TRUE 1
#define FALSE 0
#define ERROR 0
#define OK 1

typedef int QElemType_L;
typedef int Status;
typedef struct QNode
{
    QElemType_L data;
    QNode *next;
} QNode,*QueuePtr;
typedef struct
{
    QueuePtr front;
    QueuePtr rear;
} LinkQueue;
void PrintElem(QElemType_L e);
Status InitQueue_L(LinkQueue *Q);
void ClearQueue_L(LinkQueue *Q);
void DestroyQueue_L(LinkQueue *Q);
Status QueueEmpty_L(LinkQueue Q);
int QueueLength_L(LinkQueue Q);
Status GetHead_L(LinkQueue Q, QElemType_L *e);
Status EnQueue_L(LinkQueue *Q, QElemType_L e);
Status DeQueue_L(LinkQueue *Q, QElemType_L *e);
void QueueTraverse_L(LinkQueue Q, void(Visit)(QElemType_L));

Status InitQueue_L(LinkQueue *Q)
{
    QueuePtr base = (QueuePtr)malloc(sizeof(QNode));
    if(!base)
    {
        exit(OVERFLOW);
    }
    Q->front = base;
    Q->rear = Q->front;
    Q->rear->next = NULL;
}
void ClearQueue_L(LinkQueue *Q)
{
    Q->rear = Q->front->next;
    while(Q->rear)
    {
        Q->front->next = Q->rear->next;
        free(Q->rear);
        Q->rear = Q->front->next;
    }
    Q->rear = Q->front;
}
void DestroyQueue_L(LinkQueue *Q)
{
    while(Q->front)
    {
        Q->rear = Q->front->next;
        free(Q->front);
        Q->front = Q->rear;
    }
}
Status QueueEmpty_L(LinkQueue Q)
{
    return Q.front==Q.rear?TRUE:FALSE;
}
int QueueLength_L(LinkQueue Q)
{
    int count = 0;
    QueuePtr p = Q.front;
    while(p!=Q.rear)
    {
        count++;
        p=p->next;
    }
    return count;
}
Status GetHead_L(LinkQueue Q, QElemType_L *e)
{
    if(Q.front==Q.rear)
    {
        return ERROR;
    }
    *e = Q.front->next->data;
    return OK;
}
Status EnQueue_L(LinkQueue *Q, QElemType_L e)
{
    QueuePtr p = (QueuePtr)malloc(sizeof(QNode));
    if(!p)
    {
        exit(OVERFLOW);
    }
    p->data = e;
    p->next = Q->rear->next;
    Q->rear->next = p;
    Q->rear = p;
    return OK;
}
Status DeQueue_L(LinkQueue *Q, QElemType_L *e)
{
    if(Q->front==Q->rear)
    {
        return ERROR;
    }
    QueuePtr p = Q->front->next;
    *e = p->data;
    Q->front->next = p->next;
    if(p==Q->rear)
    {
        Q->rear = Q->front;
    }
    free(p);
    return OK;
}
void QueueTraverse_L(LinkQueue Q, void(Visit)(QElemType_L))
{
    if(Q.front==Q.rear)
    {
        return;
    }
    QueuePtr p = Q.front;
    while(p!=Q.rear)
    {
        Visit(p->next->data);
        p = p->next;
    }
}

void PrintElem(QElemType_L e)
{
    printf("%d ", e);
}

int main(int argc, char **argv)
{
    LinkQueue Q;
    int i;
    QElemType_L e;

    printf("▼1\n▲函数 InitQueue_L 测试...\n");  //1.函数InitQueue_L测试
    {
        printf("初始化链队 Q ...\n");
        InitQueue_L(&Q);
        printf("\n");
    }

    printf("▼4\n▲函数 QueueEmpty_L 测试...\n"); //4.函数QueueEmpty_L测试
    {
        QueueEmpty_L(Q) ? printf(" Q 为空！！\n") : printf(" Q 不为空！\n");
        printf("\n");
    }

    printf("▼7\n▲函数 EnQueue_L 测试...\n");        //7.函数EnQueue_L测试
    {
        for(i=1; i<=6; i++)
        {
            printf("元素 \"%2d\" 入队，", 2*i);
            EnQueue_L(&Q, 2*i);
            printf("（累计第 %d 个元素）...\n", QueueLength_L(Q));
        }
        printf("\n");
    }

    printf("▼9\n▲函数 QueueTraverse_L 测试...\n");//9.函数QueueTraverse_L测试
    {
        printf(" Q 中的元素为：Q = ");
        QueueTraverse_L(Q, PrintElem);
        printf("\n\n");
    }

    printf("▼8\n▲函数 DeQueue_L 测试...\n");        //8.函数DeQueue_L测试
    {
        DeQueue_L(&Q, &e);
        printf("队头元素 \"%d\" 出队...\n", e);
        printf(" Q 中的元素为：Q = ");
        QueueTraverse_L(Q, PrintElem);
        printf("\n\n");
    }

    printf("▼5\n▲函数 QueueLength_L 测试...\n");    //5.函数QueueLength_L测试
    {
        i = QueueLength_L(Q);
        printf(" Q 的长度为 %d \n", i);
        printf("\n");
    }

    printf("▼6\n▲函数 GetHead_L 测试...\n");        //6.函数GetHead_L测试
    {
        GetHead_L(Q, &e);
        printf("队头元素的值为 \"%d\" \n", e);
        printf("\n");
    }

    printf("▼3\n▲函数 ClearQueue_L 测试...\n"); //3.函数ClearQueue_L测试
    {
        printf("清空 Q 前：");
        QueueEmpty_L(Q) ? printf(" Q 为空！！\n") : printf(" Q 不为空！\n");
        ClearQueue_L(&Q);
        printf("清空 Q 后：");
        QueueEmpty_L(Q) ? printf(" Q 为空！！\n") : printf(" Q 不为空！\n");
        printf("\n");
    }

    printf("▼2\n▲函数 DestroyQueue_L 测试...\n");   //2.函数DestroyQueue_L测试
    {
        printf("销毁 Q 前：");
        Q.front!=NULL && Q.rear!=NULL ? printf(" Q 存在！\n") : printf(" Q 不存在！！\n");
        DestroyQueue_L(&Q);
        printf("销毁 Q 后：");
        Q.front!=NULL && Q.rear!=NULL ? printf(" Q 存在！\n") : printf(" Q 不存在！！\n");
        printf("\n");
    }

    return 0;
}

三、运行结果

image.png