【数据结构基础】线索二叉树（递归实现）

描述：将二叉树中结点的空左孩子指针域指向前驱结点，将空的右孩子指针域指向后继结点。
code:

#pragma warning(disable:4996)
#include<stdio.h>
#include<stdlib.h>
typedef struct TreeNode {
    char data;
    struct TreeNode *lchild, *rchild;
    int ltag, rtag;
}Tree,*BTree;
BTree Build_Tree(void) {
    BTree T;
    char ch;
    scanf("%c", &ch);
    if (ch == '#') {
        T = NULL;
    }
    else {
        T = (BTree)malloc(sizeof(Tree));
        T->data = ch;
        T->ltag = 0;
        T->rtag = 0;
        T->lchild = Build_Tree();
        T->rchild = Build_Tree();
    }
    return T;
}
//先序线索化
void Pre_Thread(BTree cur, BTree *pre) {
    if (cur && cur->ltag==0) {
        printf("%c ", cur->data);
        if (cur->lchild == NULL) {
            cur->lchild = *pre;
            (*pre)->ltag = 1;
            cur->ltag = 1;
        }
        if (cur->rchild == NULL) {
            cur->rtag = 1;
        }
        if (*pre && (*pre)->rtag == 1) {
            (*pre)->rchild = cur;
        }
        *pre = cur;
        Pre_Thread(cur->lchild, pre);
        Pre_Thread(cur->rchild, pre);
    }
}
//中序线索化
void In_Thread(BTree cur, BTree *pre) {
    if (cur) {
        In_Thread(cur->lchild, pre);
        printf("%c ", cur->data);
        if (cur->lchild==NULL) {
            cur->lchild = *pre;
            cur->ltag = 1;
        }
        if (cur->rtag == NULL) {
            cur->rtag = 1;
        }
        if (*pre && (*pre)->rtag == 1) {
            (*pre)->rchild = cur;
        }
        *pre = cur;
        In_Thread(cur->rchild, pre);
    }
}
//后序线索化
void Post_Thread(BTree cur, BTree *pre) {
    if (cur) {
        Post_Thread(cur->lchild, pre);
        Post_Thread(cur->rchild, pre);
        printf("%c ", cur->data);
        if (cur->lchild == NULL) {
            cur->lchild = *pre;
            cur->ltag = 1;
        }
        if (cur->rchild == NULL) {
            cur->rtag = 1;
        }
        if (*pre && (*pre)->rtag == 1) {
            (*pre)->rchild = cur;
        }
        *pre = cur;
    }
}
int main(void) {
    BTree T,p=NULL;
    T = Build_Tree();
    Pre_Thread(T, &p);
    //In_Thread(T, &p);
    //Post_Thread(T, &p);
    return 0;
}

跑时分别运行前序、中序、后序线索化。