将二叉搜索树转换成双向链表

题意描述：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。如下图所示：



二叉树的定义如下：

struct BinaryTreeNode {
int val;
BinaryTreeNode* pLeft;
BinaryTreeNode* pRight;
};

解题思路：二叉搜索树的中序遍历是有序的，而对于树的遍历一般都采用递归处理。这里同样，先将二叉搜索树分成左右子树，分别将左右子树排成双向链表，再连接左右子树与根结点，连接成整个双向链表
而二叉树转换成双向链表时，可以将pLeft指针表示指向上一结点的指针，将pRight指针表示指向下一结点的指针。具体实现如下：

BinaryTreeNode* ConvertToList(BinaryTreeNode* root) {
BinaryTreeNode* pNode = NULL;
ConvertToList(root, &pNode);

//经过上一步得到的双向链表，pNode每次指向链表最后一个结点，所以这里要找到头结点返回
BinaryTreeNode* head = pNode;
while (head != NULL && head->pLeft != NULL)
head = head->pLeft;
return head;
}

void ConvertToList(BinaryTreeNode* root, BinaryTreeNode**pNode) {
if (pNode == NULL)
return;
BinaryTreeNode* tmp = root;

if (tmp->pLeft != NULL)
ConvertToList(tmp->pLeft, pNode);
tmp->pLeft = *pNode;
if (*pNode != NULL)
(*pNode)->pRight = tmp;

*pNode = tmp;

if (tmp->pRight != NULL)
ConvertToList(tmp->pRight, pNode);
}

//中序遍历二叉搜索树
void ShowTree(BinaryTreeNode* root) {
if (root->pLeft != NULL)
ShowTree(root->pLeft);
cout << root->val << " ";
if (root->pRight != NULL)
ShowTree(root->pRight);
}

//双向遍历双向链表
void ShowList(BinaryTreeNode* head) {
BinaryTreeNode* pNode = head;
cout << "正向遍历双向链表：";
while (pNode->pRight != NULL) {
cout << pNode->val << " ";
pNode = pNode->pRight;
}
cout << pNode->val << endl;

cout << "反向遍历双向链表：";
while (pNode->pLeft != NULL) {
cout << pNode->val << " ";
pNode = pNode->pLeft;
}
cout << pNode->val << endl;
}

int main()
{
BinaryTreeNode* root = new BinaryTreeNode();
BinaryTreeNode* node1 = new BinaryTreeNode();
BinaryTreeNode* node2 = new BinaryTreeNode();
BinaryTreeNode* node3 = new BinaryTreeNode();
BinaryTreeNode* node4 = new BinaryTreeNode();
BinaryTreeNode* node5 = new BinaryTreeNode();
BinaryTreeNode* node6 = new BinaryTreeNode();

root->val = 10;
node1->val = 6;
node2->val = 14;
node3->val = 4;
node4->val = 8;
node5->val = 12;
node6->val = 16;

root->pLeft = node1;
root->pRight = node2;
node1->pLeft = node3;
node1->pRight = node4;
node2->pLeft = node5;
node2->pRight = node6;

cout << "初始二叉搜索树的中序遍历：";
ShowTree(root);
cout << endl;

BinaryTreeNode* head = ConvertToList(root);
ShowList(head);

return 0;
}

运行结果如下：