线索二叉树：二叉搜索树转换为双向链表

对于二叉搜索树，可以将其转换为双向链表，其中，节点的左子树指针在链表中指向前一个节点，右子树指针在链表中指向后一个节点。

 

思路：

采用递归思想，对于二叉搜索树，将左、右子树分别转换为双向链表，左子树转换所得链表的头结点即整个树的头结点，左子树转换所得链表的尾节点与根节点相邻；右子树转换所得链表的尾节点即整个树的尾节点，右子树转换所得链表的头结点与根节点相邻。

代码：

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

typedef struct TNode
{
    int value;
    TNode* lchild;
    TNode* rchild;
}TNode,*BTree;

//二叉树转换为双向链表
TNode* TreeToList(BTree tree,TNode* &lastNode)
{
    TNode* head;
    //若树为空，返回空
    if (tree == NULL)
    {
        lastNode = NULL;
        return NULL;
    }
    //若无左子树，则该根节点为链表的头结点
    if (tree->lchild==NULL)
    {
        head = tree;
    }
    //若有左子树，递归调用转换函数将左子树转换为双向链表
    //左子树转换所得链表的头结点是整个树的头结点
    //左子树链表的尾结点与根节点相邻
    else
    {
        head = TreeToList(tree->lchild,lastNode);
        tree->lchild = lastNode;
        lastNode->rchild = tree;
    }
    //若无右子树，则该根节点为链表的尾结点
    if (tree->rchild==NULL)
    {
        lastNode = tree;

    }
    //若有右子树，递归调用转换函数将左子树转换为双向链表
    //右子树转换所得链表的尾结点是整个树的尾结点
    //右子树链表的头结点与根节点相邻
    else
    {
        tree->rchild = TreeToList(tree->rchild,lastNode);
        tree->rchild->lchild = tree;
    }
    return head;
}

int main()
{
    BTree tree = (BTree)malloc(sizeof(TNode));
    tree->value = 4;
    tree->lchild = (TNode*)malloc(sizeof(TNode));
    tree->lchild->value = 2;
    tree->lchild->lchild = (TNode*)malloc(sizeof(TNode));
    tree->lchild->lchild->value = 1;
    tree->lchild->lchild->lchild = NULL;
    tree->lchild->lchild->rchild = NULL;
    tree->lchild->rchild = (TNode*)malloc(sizeof(TNode));
    tree->lchild->rchild->value = 3;
    tree->lchild->rchild->lchild = NULL;
    tree->lchild->rchild->rchild = NULL;
    tree->rchild = (TNode*)malloc(sizeof(TNode));
    tree->rchild->value = 6;
    tree->rchild->lchild = (TNode*)malloc(sizeof(TNode));
    tree->rchild->lchild->value = 5;
    tree->rchild->lchild->lchild = NULL;
    tree->rchild->lchild->rchild = NULL;
    tree->rchild->rchild = NULL;

    TNode* lastNode;
    TNode* listHead = TreeToList(tree,lastNode);
    TNode* node = listHead;
    while(node)
    {
        printf("%d ",node->value);
        node = node->rchild;
    }
    printf("\n");

    return 0;
}