POJ 2417 Discrete Logging

http://www.cnblogs.com/jianglangcaijin/archive/2013/04/26/3045795.html

给p,a,b求a^n==b%p

 #include<algorithm>
 #include<cstdio>
 #include<cmath>
 #include<cstring>
 #include<iostream>
 #include<map>
 #define ll long long
 ll p;
 std::map<ll , int> mp;
 ll Pow(ll x,ll y){
     ll res=;
     while (y){
         if (y%) res=(res*x)%p;
         x=(x*x)%p;
         y/=;
     }
     return res;
 }
 ll solve(ll a,ll b){
     a%=p;
     if (a==){
         if (b==) return ;
         else return -;
     }
     mp.clear();
     ll m=sqrt(p)+,t=,I=;
     for (int i=;i<m;i++){
         t=t*a%p;
         if (!mp[t]) mp[t]=i;
     }
     mp[]=m+;
     ll Im=Pow(a,p--m);
     for (int k=;k<m;k++){
         int i=mp[I*b%p];
         if (i!=){
             if (i==m+) i=;
             return i+k*m;
         }
         I=I*Im%p;
     }
     return -;
 }
 int main(){
     ll a,b;
     while (scanf("%lld%lld%lld",&p,&a,&b)!=EOF){
         ll ans=solve(a,b);
         if (ans==-) puts("no solution");
         else printf("%lld\n",ans);
     }
 }