Here is a quick sketch of the solutions.
In grading, I am looking only for a few things on each question,
and an overall understanding of the important concepts.


1.
   C B B C C C C E

2. (a) toString is O(N), and the other two are O(1).

   (b)
       public String toString( )
       {
           StringBuffer result = new StringBuffer( "[ " );
           for( Node<AnyType> p = header.next; p != tail; p = p.next )
               result.append( p.data + " " );
           result.append( "]" );
           return new String( result );
       }

   (c)
       public void removeFirst( )
       {
           if( header.next == tail )
               throw new IllegalStateException( );

           header.next = header.next.next;
           header.next.previous = header;
       }

   (d)
       // assume node is constructed as data/prev/next
       public void addLast( AnyType x )
       {
           tail.prev = tail.prev.next = new Node<AnyType>( x, tail.prev, tail );
       }

3.

   (a)
     public void add( String str )
     {
         Set<String> vals = theMap.get( str.toLowerCase( ) );
        
         if( vals == null )
         {
             vals = new TreeSet<>( );
             theMap.put( str.toLowerCase( ), vals  );
         }
        
         vals.add( str );
     }
    
   (b)
     public boolean contains( String str )
     {
         Set<String> vals = theMap.get( str.toLowerCase( ) );
         
         return vals != null && vals.contains( str );
     }
     
    (c)
     public boolean containsCaseInsensitive( String str )
     {
         return theMap.containsKey( str.toLowerCase( ) );
     }
 
             
              

4.
     public static File findMax( File d )
     {
         File maxFile = d;

         if( d.isDirectory( ) )
             for( File f : d.listFiles( ) )
             {
                 File thisFile  = findMax( f );
                 if( thisFile.length( ) > maxFile.length( ) )
                     maxFile = thisFile;
             }

         return maxFile;
     }