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

typedef char ElemType;

// 線索存儲標誌位
// Link(0):表示指向左右孩子的指針
// Thread(1):表示指向前驅後繼的線索
typedef enum {Link, Thread} PointerTag;



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


// 定義一個全局變量表示剛剛走過的節點
BiThrTree pre;

// 創建一棵二叉樹，約定用戶遵照前序遍歷的方式輸入數據
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);
    }
}

// 中序遍歷線索化

void InThreading(BiThrTree T){
    if ( T ) {
        InThreading(T->lchild);  // 遞歸左孩子線索化

        if( !T->lchild ){
            T->ltag = Thread;
            T
 
->lchild = pre;
        }
        if ( !pre->rchild ){
            pre->rtag = Thread;
            pre->rchild = T;
        }
        pre = T;
        InThreading(T->rchild);  // 遞歸右孩子線索化
    }
}

// 初始化一個頭指針
void InOrderThreading( BiThrTree *p, BiThrTree T){
    *p = (BiThrNode *)malloc(sizeof(BiThrNode));
    (*p)->ltag = Link;
    (*p)->rtag = Link;
    (*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);
}
// 中序遍歷二叉樹，叠代
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("中序遍歷二叉樹的結果為：");

    InOrderTraverse( P );

    printf("\n");

    return 0;
}

運行截圖

線索二叉樹的構建和遍歷------小甲魚數據結構和算法