1.


/**
 * FWP, Ausgew¦hlte Probleme aus dem ACM Programming Contest, WS10/11
 * Problem: B2 Sequence
 * Link: http://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&category=22&page=show_problem&problem=2004
 * 
 * @author Martin Lambeck
 * @version 1.0, 26.10.2010
 * 
 * Method : 
 * Status : Accepted
 * Runtime: 0.172
 */

import java.util.ArrayList;
import java.util.Scanner;


public class Main
{
	static Scanner sc = new Scanner(System.in);
	static int tcase = 1;

	public static void main (String...strings )
	{
		while(sc.hasNextInt())
			testcase();

	}

	static void testcase()
	{
		int n = sc.nextInt();
		ArrayList<Integer> lst = new ArrayList<Integer>(n);

		for (int i = 0; i < n; i++)
			lst.add(sc.nextInt());

		boolean fail = lst.get(0) < 1;

		for (int i = 1; i < n && !fail; i++)
		{
			if (!(lst.get(i-1) < lst.get(i)))
			{
				fail = true;
				break;
			}
		}

		boolean[] marked = new boolean[20001];

		for (int i = 0; i < n && !fail; i++)
		{
			for (int j = i; j < n; j++)
			{
				int sum = lst.get(i) + lst.get(j);

				if (marked[sum])
				{

					fail = true;
					break;
				}
				marked[sum] = true;
			}
		}

		System.out.printf("Case #%d: %s%n%n",
				tcase++,
				fail ? "It is not a B2-Sequence." : "It is a B2-Sequence.");

	}
}

2. 

/**
 * FWP, Ausgewählte Probleme aus dem ACM Programming Contest, SS10
 * Problem: 11063 B2-Sequence
 *
 * @author Robert Reichart
 * @version 2.0
 *
 * Status : Accepted
 * Runtime: 0.220
 */

import java.util.*;

class Main{
    public static void main(String... args){
        Scanner in = new Scanner (System.in);
        int u=0;
        while(in.hasNextInt())
        {
            u++;
            int elements = in.nextInt();
            int[] values = new int[elements];
            for (int i=0; i<elements;i++){
                values[i]=in.nextInt();
            }

            boolean fail=false;
            HashSet<Integer> sums = new HashSet<Integer>();
            //paarweise summieren
            for (int i=0; i<elements;i++){
                for (int j=i; j<elements;j++){
                    if (i>=1){if (values[i]<values[i-1]){fail=true;}}
                    if (sums.contains(values[i]+values[j]) || values[i]<1)
                    {
                        fail=true;
                        break;
                    }
                    else
                    {
                        sums.add(values[i]+values[j]);
                    }
                }
            }
            if (fail)
            {
                System.out.println("Case #" + u + ": It is not a B2-Sequence.\n");
            }else{
                System.out.println("Case #" + u + ": It is a B2-Sequence.\n");
            }
        }
    }
}