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);

}