利用链式队列实现二叉树的层次遍历（C语言）

规则：

1. 判断树是否为空，为空则返回；
2. 若不空，从树的第一层。也就是根节点开始访问。
3. 从上而下逐层遍历， 在同一层中，按从左到右的顺序对节点逐个访问。

#include<stdio.h>
#include<stdlib.h>
typedef struct tree{
    char date;
    struct tree*pLeft,*pRight;
}TREE,*PTREE;
typedef struct List{
    PTREE pBit;
    struct List *pNext;
}NODE,*PNODE;
typedef struct queue{
    PNODE front;
    PNODE rear;
}QUEUE,*PQUEUE;
void init_queue(PQUEUE);               /*初始化队列*/
void en_queue(PQUEUE,PTREE);           /*入队操作*/
PTREE out_queue(PQUEUE);               /*出队操作*/
void creat_tree(PTREE*);               /*利用先序遍历的方式创建二叉树*/
void traverse_tree(PTREE);             /*实现树的层序遍历*/
int main (void)
{
    PTREE pT = NULL;
    creat_tree(&pT);
    traverse_tree(pT);
    return 0;
}
void init_queue(PQUEUE pQ)
{
    pQ->rear = pQ->front = (PNODE)malloc(sizeof(NODE));
    if(pQ->front == NULL)
        exit(0);
    pQ->front->pNext = NULL;
    return;
}
void en_queue(PQUEUE pQ,PTREE pT)
{
    PNODE pNew = (PNODE)malloc(sizeof(NODE));
    if(pNew == NULL)
        exit(0);
    else
    {
        pNew->pBit = pT;
        pNew->pNext = NULL;
        pQ->rear->pNext = pNew;
        pQ->rear = pNew;
        return;
    }
}
PTREE out_queue(PQUEUE pQ)
{
    PNODE p = pQ->front->pNext;
    if(pQ->front == pQ->rear)   
        return NULL;
    pQ->front->pNext = p->pNext;
    if(pQ->rear == p)
        pQ->rear = pQ->front;  
    return p->pBit;
}
void creat_tree(PTREE *pT)
{
    char a;
    scanf("%c",&a);
    if(a == '#')
        *pT = NULL;
    else
    {
        (*pT) = (PTREE)malloc(sizeof(TREE));
        if((*pT) == NULL)
            exit(0);
        (*pT)->date = a;
        creat_tree(&(*pT)->pLeft);
        creat_tree(&(*pT)->pRight);
        return;
    }
}
void traverse_tree(PTREE pT)
{
    PTREE t = NULL;
    QUEUE pQ;
    init_queue(&pQ);
    if(pT != NULL)
        en_queue(&pQ,pT);                      /*若树不空，让根节点入队*/
    else
        return;
    while(pQ.front != pQ.rear)                 /*队列不空*/
    {
        t = out_queue(&pQ);                    /*让队列中元素出队*/
        printf("%c\t",t->date);
        if(t->pLeft != NULL)                   /*判断左子树是否为空*/
            en_queue(&pQ,t->pLeft);            /*不空则让它入队*/
        if(t->pRight != NULL)                  /*判断右子树是否为空*/
            en_queue(&pQ,t->pRight);           /*不空则让它入队*/
    }
    printf("\n");
    return;
}