把 $dp[i]\times \frac{i}{n}$ 移到左边去，然后就没了...
by 幽云蓝 @ 2021-07-19 16:45:46
```
n*dp[i]-i*dp[i]=(n-i)*dp[i+1]+n
dp[i]=((n-i)*dp[i+1]+n)/(n-i)
dp[i]=dp[i+1]+n/(n-i)
```
by Adis_FireDevil @ 2021-07-19 16:46:52