baby-step qiant-step

/*hdu 2815zstu 1815pku 3243hust 1234b^x=n(mod p)给定b,n,p计算经最值，or输出no solution*/#include<stdio.h>#include<math.h>#include<map>#include<stdlib.h>#include<iostream>#include<cmath>#include<time.h>using namespace std;struct z{ int re,times;}num[1000000];__int64 p,b,n;//排序int cmp(const void*a,const void*b){ if(((struct z*)a)->re!=((struct z*)b)->re) return ((struct z*)a)->re-((struct z*)b)->re; return ((struct z*)a)->times-((struct z*)b)->times;}//二分求幂__int64 mod(__int64 b,__int64 m,__int64 p){ __int64 ret=1; while(m) { if(m&1) { ret*=b; ret%=p; } b*=b; b%=p; m>>=1; } return ret;}//二分查找__int64 B_S(__int64 x,__int64 h){ __int64 l,m; __int64 best=-1; l=0; while(l<=h) { m=(l+h)>>1; if(num[m].re<x) l=m+1; else if(num[m].re>x) h=m-1; else { l=m+1; best=num[m].times; } } return best;}int main(){ __int64 i,m,j,bm,x; __int64 fac,find;__int64 T;//scanf("%d",&T); while(scanf("%I64d%I64d%I64d",&b,&p,&n)!=EOF&&(b||n||p)) {//有的时候会用到下面这个，看具体情况/*if(n>=p||p==0){puts("No Solution");continue;}*/ m=(int)(sqrt(p)); n%=p; b%=p; num[0].re=n; num[0].times=0; j=n;//保存（i,b^i）mod p for(i=1;i<=m;i++) { j*=b; j%=p; num[i].re=j; num[i].times=i; } qsort(num,m+1,sizeof(num[0]),cmp); bm=mod(b,m,p); fac=1; x=-1;//枚举(a^m)^i mod p for(i=1;i<=m;i++) { fac*=bm; fac%=p; find=B_S(fac,m); if(find+1) { x=i*m-find; break; } } if(x!=-1) {//重判，保证结果正确 fac=mod(b,x,p); if(fac==n) printf("%I64d/n",x); else if(fac*x%p==n) printf("%I64d/n",x+1); else puts("No Solution"); } else puts("No Solution"); } return 0;}