I challenged Ken Lam and won:) Challenge players from around the world! #BrainWars
http://brainwarsapp.com/

Java Primality Test

import java.util.*;
import java.math.*;

public class Solution {

	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		BigInteger n = in.nextBigInteger();
		in.close();

		if (!n.equals(new BigInteger("1")) && n.isProbablePrime(1))
			System.out.println("prime");
		else
			System.out.println("not prime");

	}
}

I saved 500MB on my phone using Files Go! Can you save more? Try the app at g.co/filesgo. #filesgo

693. Binary Number with Alternating Bits

class Solution {
    	public boolean hasAlternatingBits(int n) {
		String binary = Integer.toBinaryString(n);

		for (int i = 1; i < binary.length(); i++) {
			if (binary.charAt(i) == binary.charAt(i - 1))
				return false;
		}

		return true;
	}
}

Awesome app for brain games:

https://play.google.com/store/apps/details?id=jp.co.translimit.brainwars