https://atcoder.jp/contests/abc357/tasks/abc357_d
记 \(n\) 的位数为 \(b\),记 \(x=10^b\),则 \(ans=x^0n+x^1n+x^2n+\dots+x^{n-1}n\),提取 \(n\),得 \(n(x^0+x^1+x^2+\dots+x^{n-1})=n\dfrac{x^n-1}{x-1})\), 用快速幂求幂和逆元。
https://atcoder.jp/contests/abc357/submissions/55378135