Given a singly linked list where elements are sorted in ascending order, convert it to a height bala

将一个顺序链表转换成一棵二叉树

利用快慢指针，快指针比满指针每次多走一步

slow = slow->next;

fast = fast->next->next;

快指针走到尾，满指针走到中间点，中间点作为二叉树的根节点root，

root前的节点为左子树，root后的节点为右子树，递归

TreeNode* BST(ListNode* head, ListNode* tail)
{
	if (head == tail)
		return NULL;

	ListNode* slow = head;
	ListNode* fast = head;
	while (fast != tail && fast->next != tail)
	{
		slow = slow->next;
		fast = fast->next->next;
	}

	if (fast->next != tail)
	{
		slow = slow->next;
	}

	TreeNode* root = new TreeNode(slow->val);
	root->left = BST(head, slow);
	root->right = BST(slow->next, tail);
	return root;
}
TreeNode* sortedListToBST(ListNode *head)
{
	if (head == NULL)
		return NULL;
	else
		return BST(head, NULL);
}