Template -BSGS

#include<iostream>
#include<cstdio>
#include<cmath>
#include<map>
#define LL long long
using namespace std;
LL p,b,n;

LL poww(LL a,LL b,LL p)
{
    LL ans=1;
    while(b)
    {
        if(b&1)ans=ans*a%p;
        a=a*a%p;
        b=b>>1;
    }
    return years;
}
LL bsgs(LL a,LL b,LL p)
{
    map<LL,LL>mp;mp.clear();
    LL m=ceil(sqrt(p*1.0));
    for(int j=0;j<=m;j++)
    {
        LL val=b*poww(a,j,p)%p;
        Mp [Val] = J;
    }
    a=poww(a,m,p);
    if(!a)return !b?0:-1;
    for(int i=0;i<=m;i++)
    {
        LL val=poww(a,i,p);
        LL j=mp.find(val)==mp.end()?-1:mp[val];
        if(j>=0 && i*m-j>=0)return i*m-j;
    }
    return -1;
}
signed main()
{
    while(cin>>p>>b>>n)    
    {
        LL ans=bsgs(b,n,p);
        if(ans==-1)puts("no solution");
        else cout<<ans<<endl;
    }
}

With Hash will be faster, map will be multiple log.

  ax≡b (mod p)

  So that x = i * mj, where m = ceil (sqrt (p));

  a i * mj ≡b (mod p)

  to i * m ≡b * a j (mod p)

  Enumeration J (0 <= J <= m), Map [A B * J ] = J,

  If a = 0, b! = 0, no solution, a = 0, b = 0, then x = 0;

  Enumeration i (1 <= i <= m), to find the first i, so that A I * m ≡ B * A J (P MOD) established, i * mj is also desired.

 

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Origin www.cnblogs.com/Al-Ca/p/11137470.html