欧拉工程第58题：Spiral primes

题目链接

Java程序

package projecteuler51to60;

import java.math.BigInteger;
import java.util.Iterator;
import java.util.Set;
import java.util.TreeSet;

class level58{
    void solve0(){
/*** i = 1
        (2i-1)^2-4(i-1)   (2i-1)^2-6(i-1)
       ----------------    -----------------
        (2i-1)^2-2(i-1)      (2i-1)^2

        1+4i
***/
        int count = 0;
//        BigInteger a=BigInteger.valueOf(3);
//        BigInteger b=BigInteger.valueOf(2);

        for(int i=2;i<100000;i++){
            int a=(2*i-1)*(2*i-1)-6*(i-1);
            int b=(2*i-1)*(2*i-1)-4*(i-1);
            int c=(2*i-1)*(2*i-1)-2*(i-1);
            int num=1+4*(i-1);
            if(isPrime(a)) count++;
            if(isPrime(b)) count++;
            if(isPrime(c)) count++;
            if(1.0*count/(1.0*num)<0.1){
                System.out.println(2*i-1);
                return;
            }

        }

    }
    boolean isPrime(long num){
        if(num==2|| num==3 ||num==5 || num ==7)
            return true;
        if(num<2||num%2==0) return false;
        for(int i=3;i<Math.sqrt(num)+1;i++){
            if(num%i==0) return false;
        }
        return true;
    }
}
public class Problem58 {

    public static void main(String[] args){
        long begin= System.currentTimeMillis();
        new level58().solve0();
        long end = System.currentTimeMillis();
        long Time = end - begin;
        System.out.println("Time:"+Time/1000+"s"+Time%1000+"ms");
    }

}