Question

Can someone please tell me how to implement combination for character values stored in an array with formula nPr.
For example if I have a set of {a,b,v,f} and I want to choose 2 at a time the answer should be {a,b} {a,v} {a,f} {b,v} {b,f} {v,f}.

Or any link if this problem has a solution on the web.
Thanks.

Answer 1

Here is a general implementation:

static <T> List<List<T>> combinations( List<T> list, int n ){

    List<List<T>> result;

    if( list.size() <= n ){

        result = new ArrayList<List<T>>();
        result.add( new ArrayList<T>(list) );

    }else if( n <= 0 ){

        result = new ArrayList<List<T>>();
        result.add( new ArrayList<T>() );

    }else{

        List<T> sublist = list.subList( 1, list.size() );

        result = combinations( sublist, n );

        for( List<T> alist : combinations( sublist, n-1 ) ){
            List<T> thelist = new ArrayList<T>( alist );
            thelist.add( list.get(0) );
            result.add( thelist );
        }
    }

    return result;
}

Use it like this:

List<Character> list = new ArrayList<Character>();
list.add('a');
list.add('b');
list.add('c');
list.add('d');

List<List<Character>> combos = combinations( list, 2 );

Here’s an ideone.