Luogu4884 多少个1？（BSGS）

时间：2023-03-08 21:48:59

　　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 ;}

}

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