将二叉查找树转换为有序的双向链表

// pFirstNode：转换后双向链表的第一个节点的指针

// pLastNode：转换后双向链表的最后一个节点的指针

void Convert(BinaryTreeNode *pRoot, BinaryTreeNode *&pFirstNode, BinaryTreeNode *&pLastNode)
{
	BinaryTreeNode *pFirstLeft, *pLastLeft; // 左子树转化为双向链表后的第一个、最后一个节点的指针
	BinaryTreeNode *pFirstRight, *pLastRight; // 右子树转换为双向链表的第一个、最后一个节点的指针
	if (pRoot == NULL) {
		pFirstNode = NULL;
		pLastNode = NULL;
	}
	if (pRoot->lchild == NULL) {
		pFirstNode = pRoot;
	}
	else {
		Convert(pRoot->lchild, pFirstLeft, pLastLeft);
		pFirstNode = pFirstLeft;
		pLastLeft->rchild = pRoot;
		pRoot->lchild = pLastLeft;
	}
	if (pRoot->rchild == NULL) {
		pLastNode = pRoot;
	}
	else {
		Convert(pRoot->rchild, pFirstRight, pLastRight);
		pLastNode = pLastRight;
		pRoot->rchild = pFirstRight;
		pFirstRight->lchild = pRoot;
	}
}