規則：

1. 判斷樹是否為空，為空則返回；
2. 若不空，從樹的第一層。也就是根節點開始訪問。
3. 從上而下逐層遍歷， 在同一層中，按從左到右的順序對節點逐個訪問。

#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);               /*初始化佇列*/
void en_queue(PQUEUE,PTREE);           /*入隊操作*/
PTREE out_queue(PQUEUE);               /*出隊操作*/
void creat_tree(PTREE*);               /*利用先序遍歷的方式建立二叉樹*/
void traverse_tree(PTREE);             /*實現樹的層序遍歷*/
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);                      /*若樹不空，讓根節點入隊*/
    else
        return;
    while(pQ.front != pQ.rear)                 /*佇列不空*/
    {
        t = out_queue(&pQ);                    /*讓佇列中元素出隊*/
        printf("%c\t",t->date);
        if(t->pLeft != NULL)                   /*判斷左子樹是否為空*/
            en_queue(&pQ,t->pLeft);            /*不空則讓它入隊*/
        if(t->pRight != NULL)                  /*判斷右子樹是否為空*/
            en_queue(&pQ,t->pRight);           /*不空則讓它入隊*/
    }
    printf("\n");
    return;
}