如何把它改为先序线索？

#include
#include

typedef enum{Link,Thread} Pointag;//Link = 0表示指向左右孩子指针,Thread = 1表示指向前驱或后继的线索
typedef char ElemType;

typedef struct BiThNode
{
ElemType data;
struct BiThNode *lchild,*rchild;
Pointag ltag,rtag;
}BiThNode,*BiThTree;

BiThTree Pre; //定义一个全局变量作为节点的前驱指针

//创建一颗线索二叉树
void CreateBiThTree(BiThTree *T)
{
char c;

scanf("%c",&c);
if(' ' == c)
{
    *T = NULL;
}
else
{
    *T = (BiThTree)malloc(sizeof(BiThNode));
    (*T)->data = c;
    (*T)->ltag = (*T)->rtag = Link;                  //默认存左子树和右子树
    CreateBiThTree(&(*T)->lchild);
    CreateBiThTree(&(*T)->rchild);
}

}

//访问数结点
void visit(char c)
{
printf("%c ",c);
}

//采用中序遍历
void InTraverse(BiThTree T)
{
if(T)
{
InTraverse(T->lchild);
if(!T->lchild)

{
T->ltag = Thread;
T->lchild = Pre; //如果没有左孩子，则让左子树指针指向前一个访问的结点
}
if(!Pre->rchild)

{
Pre->rtag = Thread;
Pre->rchild = T; //前驱右孩子指针指向后继(当前结点T)
}

    Pre = T;                          //记下当前结点
    InTraverse(T->rchild);
}

}

//设置Pre指针
void InOrderThreading(BiThTree *p,BiThTree T)
{
*p = (BiThTree)malloc(sizeof(BiThNode));
(*p)->ltag = Link;
(*p)->rtag = Thread;
(*p)->rchild = *p;

if(!T) //如果树为空
{
(*p)->lchild = *p;
}
else
{
(*p)->lchild = T;
Pre = *p;
InTraverse(T);
Pre->rchild = *p;
Pre->rtag = Thread;
(*p)->rchild = Pre;
}
}

//中序遍历，非递归
void InOrderTraversing(BiThTree T)
{
BiThTree p;
p = T->lchild; //p指向根接结点
while(p != T) //空树或遍历结束时 p == T
{
while(p->ltag == Link) //遍历左子树
{
p = p->lchild;
}
visit(p->data); //访问结点数据
while( p->rtag == Thread && p->rchild != T)
{
p = p->rchild;
visit(p->data);
}
p = p->rchild;
}
}

int main()
{
BiThTree P,T = NULL;

CreateBiThTree(&T);

InOrderThreading(&P,T);

printf("中序输出结果为：");

InOrderTraversing(P);

printf("\n");

return 0;

}

http://blog.sina.com.cn/s/blog_7f0a4c7501017rrk.html

//前序线索化二叉树
void preThreading(Treep &p)
{
if(p)
{
if(!p->left)
{
p->Ltag=Thread;
p->left=pre; //前继线索
}
if(!pre->right)
{
pre->Rtag=Thread;
pre->right=p; //后继线索
}
pre=p;
if(Link==p->Ltag)
preThreading(p->left);
if(Link==p->Rtag)
preThreading(p->right);
}

    如果你能跟我解释一下
    pre=p;
    if(Link==p->Ltag) 
        preThreading(p->left);
    if(Link==p->Rtag)
        preThreading(p->right);  是什么意思，我会非常感激你