[分享]完全数的快速算法(0ms内算出前7个)

import java.math.BigInteger;
public class Perfect{
public static boolean isPrime(long n){
if(n==1) return false;
int i =2;
while(i*i<=n){
if(n%i==0) return false;
i++;
}
return true;
}
public static void main(String[] args){
long pre =System.currentTimeMillis();
for(int n=1;n<=20;n++){
if(!isPrime(n)) continue;
long m =(1<<n) -1; //2^n -1
if(!isPrime(m)) continue;
long perfect =m<<(n-1);
System.out.println(\"完全数: \"+perfect);
}
System.out.println(\"耗时: \"+(System.currentTimeMillis()-pre)+\"ms\");
}
}