poj2417 Baby-StepGiant-StepAlgorithm a^x=b%P

#include <iostream>
#include <algorithm>
#include <string.h>
#include <cstdio>
#include <cmath>
using namespace std;
typedef long long LL;
struct Node{
  int idx;
  LL val;
  Node(int cidx=0, LL cval=0){
     idx=cidx;
     val=cval;
  }
  bool operator <(const Node &rhs)const{
     return val<rhs.val||( val == rhs.val && idx<rhs.idx );
  }
}P[1000000];
LL pow_mod(LL a, LL n, LL mod)
{
     LL ans=1;
     while(n)
     {
         if(n&1)ans=(ans*a)%mod;
         n>>=1;
         a=(a*a)%mod;
     }
     return ans;
}
int bitsearch(LL d,int n)
{
     int L=0,R=n-1;
     while(L<=R)
     {
         int mid=(L+R)>>1;
         if(P[mid].val==d) return P[mid].idx;
         if(P[mid].val<d) L=mid+1;
         else R=mid-1;
     }
     return -1;
}
LL log_mod(LL a, LL b, LL n)
{
    LL m,v,e=1;
    m=(sqrt(n+0.5))+1;
    v=pow_mod(a,n-m-1,n);
    P[0]=Node(0,1);
    for(int i=1; i<m; i++)
    {
         e=(e*a)%n;
         P[i]=Node(i,e);
    }
    sort(P,P+m);
    int cnt=1;
    for(int i=0; i<m; i++)
        if(P[i].val!=P[cnt-1].val) P[cnt++]=P[i];
    for(int i=0; i<m; i++)
    {
          int loc=bitsearch(b,cnt);
          if(loc!=-1){
            return i*m+loc;
          }
          b=(b*v)%n;
    }

    return -1;
}
int main()
{
    LL P,B,N;
    while(scanf("%I64d%I64d%I64d",&P,&B,&N)==3)
    {
        LL d =log_mod(B,N,P);
        if(d==-1)puts("no solution");
        else printf("%I64d
",d);
    }
    return 0;
}