1.把二元查找树转变成排序的双向链表

///////////////////////////////////////////////////////////////////////
// Covert a sub binary-search-tree into a sorted double-linked list
// Input: pNode -           the head of the sub tree
//        pLastNodeInList - the tail of the double-linked list
///////////////////////////////////////////////////////////////////////
void ConvertNode(BSTreeNode* pNode, BSTreeNode*& pLastNodeInList)
{
    if(pNode == NULL)
        return;

    BSTreeNode *pCurrent = pNode;

    // Convert the left sub-tree
    if (pCurrent->m_pLeft != NULL)
        ConvertNode(pCurrent->m_pLeft, pLastNodeInList);

    // Put the current node into the double-linked list
    pCurrent->m_pLeft = pLastNodeInList; 
    if(pLastNodeInList != NULL)
        pLastNodeInList->m_pRight = pCurrent;

    // Update the last node in list
    pLastNodeInList = pCurrent;

    // Convert the right sub-tree
    if (pCurrent->m_pRight != NULL)
        ConvertNode(pCurrent->m_pRight, pLastNodeInList);
}

///////////////////////////////////////////////////////////////////////
// Covert a binary search tree into a sorted double-linked list
// Input: pHeadOfTree - the head of tree
// Output: the head of sorted double-linked list
///////////////////////////////////////////////////////////////////////
BSTreeNode* Convert_Solution1(BSTreeNode* pHeadOfTree)
{
    BSTreeNode *pLastNodeInList = NULL;
    ConvertNode(pHeadOfTree, pLastNodeInList);
    if (pLastNodeInList !=NULL)
        pLastNodeInList->m_pRight = NULL;

    // Get the head of the double-linked list
    BSTreeNode *pHeadOfList = pLastNodeInList;
    while(pHeadOfList && pHeadOfList->m_pLeft)
        pHeadOfList = pHeadOfList->m_pLeft;

    return pHeadOfList;
}