Poj1881: prime factor decomposition + prime number testing

Good beginners

First, test whether it is a prime number. If not, perform prime factor decomposition. For more information about the algorithm, see Miller Robin and Pollard rock.

AC code

#include <iostream>#include<stdio.h>#include<algorithm>#include<math.h>using namespace std;long long ans;long long gcd(long long a,long long b){    return b?gcd(b,a%b):a;}long long random(long long n){    return (long long)(rand()%(n-1)+1);}long long multi(long long a,long long b,long long m)//a*b%m{    long long res=0;    while(b>0)    {        if(b&1)            res=(res+a)%m;        b>>=1;        a=(a<<1)%m;    }    return res;}long long quickmod(long long a,long long b,long long m) //a^b%m{    long long res=1;    while(b>0)    {        if(b&1)            res=multi(res,a,m);        b>>=1;        a=multi(a,a,m);    }    return res;}int check(long long a,long long n,long long x,long long t){    long long res=quickmod(a,x,n);    long long last=res;    for(int i=1;i<=t;i++)    {        res=multi(res,res,n);        if(res==1&&last!=1&&last!=n-1) return 1;        last=res;    }    if(res!=1) return 1;    return 0;}int primetest(long long n){    if(n<2)return 0;    if(n==2)return 1;    if((n&1)==0) return 0;    long long x=n-1;    long long t=0;    while((x&1)==0){x>>=1;t++;}    for(int i=0;i<20;i++)    {        long long a=random(n);        if(check(a,n,x,t))            return 0;    }    return 1;}long long pollardrho(long long n,long long c){    long long x,y,d,i,k;    i=1;k=2;    x=random(n);    y=x;    while(1)    {        i++;        x=(multi(x,x,n)+c)%n;        long long tmp=y-x>=0?y-x:x-y;        d=gcd(tmp,n);        if(d>1&&d<n)            return d;        if(y==x)            return n;        if(i==k)        {            y=x;            k+=k;        }    }}void findfac(long long n){    if(n==1)        return;    if(primetest(n))    {        ans=min(ans,n);        return;    }    long long p=n;    while(p>=n)        p=pollardrho(n,random(n-1));    findfac(p);    findfac(n/p);}int main(){    int t;    long long n;    scanf("%d",&t);    while(t--)    {        scanf("%I64d",&n);        if(primetest(n))        {            puts("Prime");            continue;        }        ans=n;        findfac(n);        printf("%I64d\n",ans);    }    return 0;}

 

Poj1881: prime factor decomposition + prime number testing