import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.TreeSet;
import java.util.HashSet;
import java.util.Collection;

public class Day17
{
    public static void populate( Collection<Integer> list, int N )
    {
        for( int i = 0; i < N; i++ )
            list.add( i * 2 );

        for( int i = 0; i < N; i++ )
            if( !list.contains( i * 2 ) || list.contains( i * 2 + 1 ) )
                System.out.println( "OOPS!" );
    }
    public static void main( String [ ] args )
    {
        final int N = 20;
        Collection<Integer> list = new TreeSet<Integer>( );

        long start = System.currentTimeMillis( );
        populate( list, N );


        long end = System.currentTimeMillis( );

        System.out.println( list );
        list.remove( 4 );
        
        System.out.println( list );
        System.out.println( list.size( ) );
        System.out.println( "Elapsed time is " + ( end - start ) + "millisec" );
    }

}