线索二叉树里不明白的地方！

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

typedef char ElemType;

// 线索存储标志位
// Link(0)：表示指向左右孩子的指针
// Thread(1)：表示指向前驱后继的线索
typedef enum {Link, Thread} PointerTag;

typedef struct BiThrNode
{
        char data;
        struct BiThrNode *lchild, *rchild;
        PointerTag ltag;
        PointerTag rtag;
} BiThrNode, *BiThrTree;

// 全局变量，始终指向刚刚访问过的结点
BiThrTree pre;

// 创建一棵二叉树，约定用户遵照前序遍历的方式输入数据
void CreateBiThrTree( BiThrTree *T )
{
        char c;

        scanf("%c", &c);
        if( ' ' == c )
        {
                *T = NULL;
        }
        else
        {
                *T = (BiThrNode *)malloc(sizeof(BiThrNode));
                (*T)->data = c;
                (*T)->ltag = Link;
                (*T)->rtag = Link;

                CreateBiThrTree(&(*T)->lchild);
                CreateBiThrTree(&(*T)->rchild);
        }
}

// 中序遍历线索化
void InThreading(BiThrTree T)
{
        if( T )
        {
                InThreading( T->lchild );                // 递归左孩子线索化

                if( !T->lchild )        // 如果该结点没有左孩子，设置ltag为Thread，并把lchild指向刚刚访问的结点。
                {
                        T->ltag = Thread;
                        T->lchild = pre;
                }

                if( !pre->rchild )
                {
                        pre->rtag = Thread;
                        pre->rchild = T;
                }

                pre = T;

                InThreading( T->rchild );                // 递归右孩子线索化
        }
}

void InOrderThreading( BiThrTree *p, BiThrTree T )
{
        *p = (BiThrTree)malloc(sizeof(BiThrNode));
        (*p)->ltag = Link;
        (*p)->rtag = Thread;
        (*p)->rchild = *p;
        if( !T )
        {
                (*p)->lchild = *p;
        }
        else
        {
                (*p)->lchild = T;
                pre = *p;
                InThreading(T);
                pre->rchild = *p;
                pre->rtag = Thread;
                (*p)->rchild = pre;
        }
}

void visit( char c )
{
        printf("%c", c);
}

// 中序遍历二叉树，非递归
void InOrderTraverse( BiThrTree T )
{
        BiThrTree p;
        p = T->lchild;

        while( p != T )
        {
                while( p->ltag == Link )
                {
                        p = p->lchild;
                }
                visit(p->data);

                while( p->rtag == Thread && p->rchild != T )
                {
                        p = p->rchild;
                        visit(p->data);
                }

                p = p->rchild;
        }
}

int main()
{
        BiThrTree p, T = NULL;

        CreateBiThrTree( &T );

        InOrderThreading( &p, T );

        printf("the result: \n");

        InOrderTraverse( p );

        printf("\n");

        return 0;
}