Luogu4884 多少个1？（BSGS）

　　11……1(n个)=99……9(n个)/9=(10ⁿ-1)/9。

　　那么显然就是求离散对数了，BSGS即可。

#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
#include<map>
using namespace std;
#define ll long long
ll read()
{
    ll x=,f=;char c=getchar();
    while (c<''||c>'') {if (c=='-') f=-;c=getchar();}
    while (c>=''&&c<='') x=(x<<)+(x<<)+(c^),c=getchar();
    return x*f;
}
ll k,m,block,a[];
map<ll,ll> f;
ll ksc(ll a,ll b)
{
    ll t=a*b-(ll)((long double)a*b/m+0.5)*m;
    return t<?t+m:t;
}
int main()
{
#ifndef ONLINE_JUDGE
    freopen("a.in","r",stdin);
    freopen("a.out","w",stdout);
    const char LL[]="%I64d\n";
#else
    const char LL[]="%lld\n";
#endif
    k=read(),m=read();
    k=(k*+)%m;
    block=sqrt(m);f[a[]=k]=;ll s=;
    for (int i=;i<=block;i++) f[a[i]=a[i-]*%m]=i+,s=s*%m;
    ll t=s;
    for (ll i=block;i<=m+block;i+=block,s=ksc(s,t))
    if (f[s]) {cout<<i-f[s]+;return ;}
}