In mathematics you don't understand things, you just get used to them.

标签 probablities 下的文章

link.对于 pass 1, 你把他考虑成 $\frac{\sum x}{i}$ 的形式, 于是每次操作的贡献就是 $\frac{2}{i}$, 那么答案就是 $\sum_{i=2}^n\fr...