线索化二叉树

    线索化二叉树指的是将二叉树中的结点进行逻辑意义上的“重排列”，使其可以线性的方式访问

每一个结点。二叉树线索化之后每个结点都有一个线性下标，通过这个下标可以快速访问结点，而不

需要遍历二叉树。线索化二叉树有两种方法：

1、利用结点中的空指针域，使其指向后继结点。

算法思想：

    

    初始化位置指针：p = NULL

    前序遍历二叉树：若p不为空，将p->left指向当前结点，并将p置为NULL；若当前结点的左子树为空时，将p指向当前结点

    

2、利用线性表保存二叉树的遍历顺序

算法思想：

    初始化顺序表s1;

    前序遍历二叉树：遍历过程中将当前结点插入顺序表s1（借助之前实现的顺序表）

/**实现代码ThreadBTree.c**/
#include <stdio.h>
#include <stdlib.h>
#include "BTree.h"
#include "SeqList.h"

/* run this program using the console pauser or add your own getch, system("pause") or input loop */

struct Node
{
    BTreeNode header;
    char v;
};

void printf_data(BTreeNode* node)
{
    if( node != NULL )
    {
        printf("%c", ((struct Node*)node)->v);
    }
}


void thread_via_left(BTreeNode* root, BTreeNode** pp)
{
    if( (root != NULL) && (pp != NULL) )
    {
        if( *pp != NULL )
        {
            (*pp)->left = root;

            *pp = NULL;

        }

        if( root->left == NULL )
        {
            *pp = root;
        }

        thread_via_left(root->left,pp);
        thread_via_left(root->right,pp);
    }
}

void thread_via_list(BTreeNode* root, SeqList* list)
{
    if( (root != NULL) && (list != NULL))
    {
        SeqList_Insert(list,(SeqListNode*)root,SeqList_Length(list));
        thread_via_list(root->left,list);
        thread_via_list(root->right,list);
    }

}

int main(int argc, char *argv[])
{
    BTree* tree = BTree_Create();
    BTreeNode* current = NULL;
    BTreeNode* p = NULL;
    SeqList* list = NULL;
    int i = 0 ;

    struct Node n1 = {{NULL, NULL}, 'A'};
    struct Node n2 = {{NULL, NULL}, 'B'};
    struct Node n3 = {{NULL, NULL}, 'C'};
    struct Node n4 = {{NULL, NULL}, 'D'};
    struct Node n5 = {{NULL, NULL}, 'E'};
    struct Node n6 = {{NULL, NULL}, 'F'};

    BTree_Insert(tree, (BTreeNode*)&n1, 0, 0, 0);
    BTree_Insert(tree, (BTreeNode*)&n2, 0x00, 1, 0);
    BTree_Insert(tree, (BTreeNode*)&n3, 0x01, 1, 0);
    BTree_Insert(tree, (BTreeNode*)&n4, 0x00, 2, 0);
    BTree_Insert(tree, (BTreeNode*)&n5, 0x02, 2, 0);
    BTree_Insert(tree, (BTreeNode*)&n6, 0x02, 3, 0);

    printf("Full Tree: \n");

    BTree_Display(tree, printf_data, 4, '-');

    printf("Thread via list: \n");

    list = SeqList_Create(BTree_Count(tree));

    thread_via_list(BTree_Root(tree),list);

    for(i=0; i<SeqList_Length(list);i++)
    {
        printf("%c,", ((struct Node*)SeqList_Get(list,i))->v);
    }

    printf("\n");

    printf("Thread via Left: \n");

    current = BTree_Root(tree);

    thread_via_left(current,&p);

    while( current != NULL )
    {
         printf("%c,", ((struct Node*)current)->v);

         current = current->left;
    }

    printf("\n");

    BTree_Destroy(tree);
    printf("please enter to continue...");
    getchar();
    return 0;
}

小结：

1>利用结点空指针线索化的方法会破坏树的结构；利用结点空指针线索化二叉树之后不能够再恢复，

这两个问题可以在树结点中加入一个线索化指针而得以解决，然而线索化指针的加入又会浪费内存空

间，不够灵活；

2>链表线索化方法不会破坏树的结构不需要时线索化时销毁链表即可；链表线索化方法可以很容易的

以任何一种遍历顺序对二叉树进行线索化。