Question

This was an interview question:

Given an amount, say $167.37 find all the possible ways of generating the change for this amount using the denominations available in the currency?

Anyone who could think of a space and time efficient algorithm and supporting code, please share.

Here is the code that i wrote (working) . I am trying to find the running time of this, any help is appreciated

    import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.Map;


public class change_generation {

    /**
     * @param args
     */

    public static void generatechange(float amount,LinkedList<Float> denominations,HashMap<Float,Integer> useddenominations)
    {
        if(amount<0)
            return;
        if(amount==0)
        {
            Iterator<Float> it = useddenominations.keySet().iterator();
            while(it.hasNext())
            {
                Float val = it.next();
                System.out.println(val +" :: "+useddenominations.get(val));
            }

            System.out.println("**************************************");
            return;
        }

        for(Float denom : denominations)
        {

            if(amount-denom < 0)
                continue;

            if(useddenominations.get(denom)== null)
                useddenominations.put(denom, 0);

            useddenominations.put(denom, useddenominations.get(denom)+1);
            generatechange(amount-denom, denominations, useddenominations);
            useddenominations.put(denom, useddenominations.get(denom)-1);
        }

    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        float amount = 2.0f;
        float nikle=0.5f;
        float dollar=1.0f;
        float ddollar=2.0f;

        LinkedList<Float> denominations = new LinkedList<Float>();


        denominations.add(ddollar);
        denominations.add(dollar);
        denominations.add(nikle);

        HashMap<Float,Integer> useddenominations = new HashMap<Float,Integer>();
        generatechange(amount, denominations, useddenominations);
    }

}

Answer 1

import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.Map;

public class change_generation {

  static int jj=1;

  public static void generatechange(float amount,LinkedList<Float> denominations, 
                                  HashMap<Float,Integer> useddenominations) {
    if(amount<0)
        return;
    if(amount==0) {
        Iterator<Float> it = useddenominations.keySet().iterator();
        while(it.hasNext()) {
            Float val = it.next();
            System.out.println(val +" :: "+useddenominations.get(val));
        }
        System.out.println("**************************************");

        return;
    }

    for(Float denom : denominations) {
        if(amount-denom < 0)
            continue;
        if(useddenominations.get(denom)== null)
            useddenominations.put(denom, 0);

        useddenominations.put(denom, useddenominations.get(denom)+1);
        generatechange(amount-denom, denominations, useddenominations);
        useddenominations.put(denom, useddenominations.get(denom)-1);
    }
  }

  public static void main(String[] args) {
    float amount = 2.0f;
    float nikle=0.25f;
    float dollar=1.0f;
    float ddollar=2.0f;

    LinkedList<Float> denominations = new LinkedList<Float>();

    denominations.add(ddollar);
    denominations.add(dollar);
    denominations.add(nikle);

    HashMap<Float,Integer> useddenominations = new HashMap<Float,Integer>();
    generatechange(amount, denominations, useddenominations);
  }
}