bzoj 3239 poj 2417 BSGS

BSGS算法，预处理出ϕ(c)−−−−√内的a的幂，每次再一块一块的往上找，转移时将b乘上逆元，哈希表里O(1)查询即可

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#include<cmath>
#include<map>
#define LL long long
long long a,b,c,m;
bool bo=0;
std::map<long long,int> pp;
std::map<long long,bool> vis;
LL exgcd(LL o,LL p,LL &x,LL &y){
    if(p==0){
        x=1;y=0;
        return o;
    }
    LL gcd=exgcd(p,o%p,x,y);
    LL t=x;
    x=y;
    y=t-(o/p)*x;
    return gcd;
}
int main(){
    while(scanf("%lld%lld%lld",&c,&a,&b)==3){
        pp.clear(); vis.clear(); bo=0;
        if(a%c==0){printf("no solution\n");continue;}
        m=(LL)ceil(sqrt((double)c));
        LL now=1;
        pp[now]=0; vis[now]=1;
        for(int i=1;i<m;i++){
            now=(now*a)%c;
            if(!vis[now]){vis[now]=1;pp[now]=i;}
        }
        now=(now*a)%c;
        LL x,y;
        LL d=exgcd(now,c,x,y);
        x=(x%c+c)%c;
        for(int i=0;i<=m;i++){
            if(vis[b]){
                printf("%lld\n",i*m+pp[b]);
                bo=1; break;
            }
            b=(b*x)%c;
        }
        if(bo==1)continue;
        printf("no solution\n");
    }
    return 0;
}