Spotify 'Reversed Binary' test program solved (https://www.spotify.com/us/jobs/tech/reversed-binary/)

ReversedBinaryNumbers.java

import java.util.Scanner;

public class ReversedBinaryNumbers {
    public static void main(String[] args) {
   	if (args.length == 1) {
		Scanner scanner=new Scanner(args[0]);
		if (scanner.hasNextLong()) {
			long l=scanner.nextLong();
			if ((l>0)&&(l<1000000001)) {System.out.println(Long.parseLong(new StringBuilder(Long.toBinaryString(l)).reverse().toString(),2));}
		}
	}
    }
}

ReversedBinaryNumbers2.java

// Recursive solution
import java.util.Scanner;
 
public class ReversedBinaryNumbers2 {
    	private static String decimal2BinaryReversed(String s) {
		if (Long.parseLong(s)<2) {return s;}
		else {return ((Long.parseLong(s)%2)+decimal2BinaryReversed(String.valueOf(Long.parseLong(s)/2)));}
	}
	
	private static long binary2Decimal(String s) {
		if (s.length()<2) {return Long.parseLong(s);}
		else {return (long) ((Long.parseLong(s.substring(0,1))*Math.pow(2,s.length()-1))+binary2Decimal(s.substring(1)));}
	}
	
	public static void main(String[] args) {	
		if (args.length == 1) {
		    Scanner scanner=new Scanner(args[0]);
		    if (scanner.hasNextLong()) {
		    	long l=scanner.nextLong();
		    	if ((l>0)&&(l<1000000001)) {System.out.println(binary2Decimal(decimal2BinaryReversed(args[0])));}
		    }
		}
	}
}