Print all unique combination of factors of a given number

Question

What is the most efficient algorithm to print all unique combinations of factors of a positive integer. For example if the given number is 24 then the output should be

Answer 1

This code find all the factors of a number, sort them (locally and globally):

public class PrimeFactors {

   final SortedSet< List< Integer >> _solutions = new TreeSet<>(
      new Comparator>(){
         @Override
         public int compare( List left, List right ) {
            int count = Math.min( left.size(), right.size());
            for( int i = 0; i < count; ++i ) {
               if( left.get(i) < right.get(i)) {
                  return -1;
               }
               if( left.get(i) > right.get(i)) {
                  return +1;
               }
             }
            return left.size() - right.size();
         }});

   public SortedSet< List< Integer >> getPrimes( int num ) {
      _solutions.clear();
      getPrimes( num, new LinkedList< Integer >());
      return _solutions;
   }

   private void getPrimes( int num, List< Integer > solution ) {
      for( int i = num - 1; i > 1; --i ) {
         if(( num % i ) == 0 ) {
            int temp = num / i;
            List< Integer > s = new LinkedList<>();
            s.addAll( solution );
            s.add( temp );
            s.add( i );
            Collections.sort( s );
            if( _solutions.add( s )) { // if not already found
               s = new LinkedList<>();
               s.addAll( solution );
               s.add( temp );
               getPrimes( i, s );
             }
         }
      }
   }
   public static void main( String[] args ) {
      SortedSet< List< Integer >> solutions =
         new PrimeFactors().getPrimes( 24 );
      System.out.println( "Primes of 24 are:" );
      for( List< Integer > l : solutions ) {
         System.out.println( l );
      }
   }
}

Outputs:

Primes of 24 are:
[2, 2, 2, 3]
[2, 2, 6]
[2, 3, 4]
[2, 12]
[3, 8]
[4, 6]