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; 
}