有序单链表转换为平衡二叉树。

TreeNode *buildTree(ListNode *&list,int n)
{
	if (n == 0)
		return NULL;
	
	TreeNode *root = new TreeNode(0);
	root->left = buildTree(list,n / 2);
	root->val = list->val;
	list = list->next;
	root->right = buildTree(list,n - n / 2 - 1);
	return root;
}

TreeNode* sortedListToBST(ListNode* head) 
{
	int size = 0;
	ListNode *p = head;
	while (p)
	{
		size++;
		p = p->next;
	}
	p = head;
	ListNode *list = head;
	return buildTree(list,size);

}