Integer to Roman

public class Solution {
    ArrayList< HashMap<Integer, String> > romanVsInt;
    
    public Solution(){
        initRomanVsInt();
    }
    
    public void initRomanVsInt(){
        romanVsInt = new ArrayList< HashMap< Integer, String > >();    
        HashMap< Integer, String > unit = new HashMap< Integer, String >();
        unit.put( 1, "I" );
        unit.put( 2, "II" );
        unit.put( 3, "III" );
        unit.put( 4, "IV" );
        unit.put( 5, "V" );
        unit.put( 6, "VI" );
        unit.put( 7, "VII" );
        unit.put( 8, "VIII" );
        unit.put( 9, "IX" );
        romanVsInt.add( unit );    //add the unit digit transform table
        HashMap< Integer, String > ten = new HashMap< Integer, String >();
        ten.put( 1, "X" );
        ten.put( 2, "XX" );
        ten.put( 3, "XXX" );
        ten.put( 4, "XL" );
        ten.put( 5, "L" );
        ten.put( 6, "LX" );
        ten.put( 7, "LXX" );
        ten.put( 8, "LXXX" );
        ten.put( 9, "XC" );
        romanVsInt.add( ten );    //add the ten‘ digit transform table
        HashMap< Integer, String > hundred = new HashMap< Integer, String >();
        hundred.put( 1, "C" );
        hundred.put( 2, "CC" );
        hundred.put( 3, "CCC" );
        hundred.put( 4, "CD" );
        hundred.put( 5, "D" );
        hundred.put( 6, "DC" );
        hundred.put( 7, "DCC" );
        hundred.put( 8, "DCCC" );
        hundred.put( 9, "CM" );
        romanVsInt.add( hundred );    //add the hundred‘ digit transform table
        HashMap< Integer, String > thousand = new HashMap< Integer, String >();
        thousand.put( 1, "M");
        thousand.put( 2, "MM");
        thousand.put( 3, "MMM" );
        romanVsInt.add( thousand );    //add the thousands‘ digit transform table
    }
    
    public String intToRoman( int num ){
        int[] toPart = new int[4];
        int i=0;
        while( num!=0 ){
            toPart[i++] = num%10;
            num /= 10;
        }
        
        StringBuilder res = new StringBuilder();
        for( i--; i>=0;  i-- ){
            if( toPart[i] == 0 )    continue;
            res.append( romanVsInt.get(i).get( toPart[i]) );
        }
        
        return res.toString();
    }
}