二叉树：前序遍历，中序遍历，后序遍历和层序遍历

#include <iostream>
using namespace std;

typedef struct BiTNode {//二叉树结构体
    int data;
    struct BiTNode* lchild, * rchild;
}BiTNode, * BiTree;

typedef struct LinkNode {//下边两个结构体是队列
//由于下一行用到了BiTree结构体，所以应该把二叉树结构体BiTNode放在前，队列结构体放在后，否则VS2019报错
     BiTree tree;
    struct LinkNode *next;
};
typedef struct LinkQueue {
    LinkNode * front, * rear;
};

void initQueue(LinkQueue &q)
{
    q.front = q.rear = (LinkNode*)malloc(sizeof(LinkNode));
    q.front->next = NULL;
}
bool isEmpty(LinkQueue q)
{
    return q.front == q.rear ? true : false;
}
void EnQueue(LinkQueue &q, BiTree t)
{
    //cout << "this is EnQueue:" << t->data <<",lchild:"<<t->lchild->data<<",rchild:"<<t->rchild->data<< endl;
    LinkNode *p = (LinkNode *)malloc(sizeof(LinkNode));
    p->tree = t;
    p->next = NULL;
    q.rear->next = p;
    q.rear = p;
    //cout << "in " << t->data << endl;
}
BiTree DeQueue(LinkQueue& q)
{
    BiTree t;
    LinkNode* p = q.front->next;
    t = p->tree;
    //cout << endl;
    //cout << "quit " << t->data << endl;
    q.front->next = p->next;
    if (q.rear == p) q.rear = q.front;  
//这里不要写成q.front=q.rear，感谢Mr.Sunshine帮我改bug，
    free(p);
    return t;
}

BiTree init()//先序输入一个二叉树，输入0代表该树为空
{
    int dat;
    cin >> dat;
    if (dat == 0) return NULL;
    else
    {
        BiTree t = (BiTree)malloc(sizeof(BiTNode));
        t->data = dat;
        t->lchild = init();
        t->rchild = init();
        return t;
    }
}
void PreOrder(BiTree t)//先序遍历
{
    if (t != NULL)
    {
        cout << t->data << " ";
        PreOrder(t->lchild);
        PreOrder(t->rchild);
    }
}
void InOrder(BiTree t)//中序遍历
{
    if (t != NULL)
    {
        InOrder(t->lchild);
        cout << t->data << " ";
        InOrder(t->rchild);
    }
}
void PostOrder(BiTree t)//后序遍历
{
    if (t != NULL)
    {
        PostOrder(t->lchild);
        PostOrder(t->rchild);
        cout << t->data << " ";
    }

}
void LevelOrder(BiTree t)//层序遍历，用队列辅助
{
    LinkQueue q;
    initQueue(q);
    BiTree temp;
    if (t == NULL) {
        cout << "树为空" << endl;
    }
    else
    {
        EnQueue(q,t);
        while (!isEmpty(q))
        {
            temp=DeQueue(q);
            cout <<temp->data<<" ";
            if (temp->lchild != NULL)
            {
                EnQueue(q, temp->lchild);
            }
            if (temp->rchild != NULL)
            {
                EnQueue(q, temp->rchild);
            }
        }
    }
}
int main(void)
{
    BiTree t = init();//建立二叉树，并输入
    cout <<"前序遍历：";
    PreOrder(t);
    cout << endl;
    cout << "中序遍历：";
    InOrder(t);
    cout << endl;
    cout << "后序遍历：";
    PostOrder(t);
    cout << endl;
    cout << "层次遍历：";
    LevelOrder(t);
    cout << endl;
}