二叉搜索树变双向链表

//输入一棵二叉搜索树，将该二叉搜索树转换成一个排序的双向链表。
//要求不能创建任何新的结点，只能调整树中结点指针的指向。
#include "stdlib.h"

struct BinaryTreeNode
{
int m_nValue;
BinaryTreeNode* m_pLeft;
BinaryTreeNode* m_pRight;
};
//第一种需要额外空间，先中序，再连。

//第二种，不用额外空间。
BinaryTreeNode* convert(BinaryTreeNode* pRootOfTree)
{
if (pRootOfTree == NULL)
return NULL;
BinaryTreeNode* leftList = convert(pRootOfTree->m_pLeft);

if (leftList != NULL)
{
BinaryTreeNode* leftLast = leftList;
while (leftLast->m_pRight != NULL)
leftLast = leftLast->m_pRight;

leftLast->m_pRight = pRootOfTree;
pRootOfTree->m_pLeft = leftLast;
}
else
{
pRootOfTree->m_pLeft = NULL;
leftList = pRootOfTree;
}

BinaryTreeNode* right = convert(pRootOfTree->m_pRight);

if (right != NULL)
{
right->m_pLeft = pRootOfTree;
}
pRootOfTree->m_pRight = right;

return leftList;
}

//左指针变前向指针，右指针变后向指针