### Hexagonal tile differences题号：128 难度： 55 中英对照

A hexagonal tile with number 1 is surrounded by a ring of six hexagonal tiles, starting at "12 o'clock" and numbering the tiles 2 to 7 in an anti-clockwise direction.

New rings are added in the same fashion, with the next rings being numbered 8 to 19, 20 to 37, 38 to 61, and so on. The diagram below shows the first three rings.

By finding the difference between tile n and each of its six neighbours we shall define PD(n) to be the number of those differences which are prime.

For example, working clockwise around tile 8 the differences are 12, 29, 11, 6, 1, and 13. So PD(8) = 3.

In the same way, the differences around tile 17 are 1, 17, 16, 1, 11, and 10, hence PD(17) = 2.

It can be shown that the maximum value of PD(n) is 3.

If all of the tiles for which PD(n) = 3 are listed in ascending order to form a sequence, the 10th tile would be 271.

Find the 2000th tile in this sequence.

### Code


public final class p128 {

public static void main(String[] args) {
long start=System.nanoTime();
String result = run();
long end=System.nanoTime();
System.out.println(result);
System.out.println( (end-start)/1000000 + "ms" );
}

public static String run(){
return solve(2000)+"";
}

public static long solve(int N) {
for (long n = 2, p = 8, m = 6;; m += 6, p += m) {
if (isPrime(m + 5) && isPrime(m + 7) && isPrime(2 * m + 17)
&& (++n == N))
return p;
if (isPrime(m + 5) && isPrime(m + 11) && isPrime(2 * m + 5)
&& (++n == N))
return p + m + 5;
}
}

// prime number checking function
public static boolean isPrime(long n) {
if (n < 2)
return false;
if (n == 2 || n == 3)
return true;
if (n % 2 == 0 || n % 3 == 0)
return false;
long sqrtN = (long) Math.sqrt(n) + 1;
for (long i = 6L; i <= sqrtN; i += 6) {
if (n % (i - 1) == 0 || n % (i + 1) == 0)
return false;
}

return true;
};
}
14516824220
145ms