Coin Sums: https://projecteuler.net/problem=31

CoinSums.java

enum Coin {
  ONE_P(1),
  TWO_P(2),
  FIVE_P(5),
  TEN_P(10),
  TWENTY_P(20),
  FIFTY_P(50),
  ONE_L(100),
  TWO_L(200);
  
  public int value;
  
  Coin(final int value) {
    this.value = value;
  }
}

class Purse {
  public final Map<Coin, Integer> coins;
  
  public Purse() {
    this.coins =  new ConcurrentHashMap<Coin, Integer>();
  }
  
  public Purse(final Purse rhs) {
    this.coins = new ConcurrentHashMap<Coin, Integer>(rhs.coins);
  }
  
  public Purse putInto(final Coin coin) {
    final Integer current = coins.getOrDefault(coin, 0);
    
    coins.put(coin, current + 1);
    
    return this;
  }
}

public class Solution {
  final Map<Integer, Set<Purse>> memo = new ConcurrentHashMap<Integer, Set<Purse>>();
  
  private Set<Purse> getPurses(final Purse purse, final int amount) {
    final Set<Purse> pursesMemo = memo.get(amount);
    if (pursesMemo != null) {
      return pursesMemo;
    }
    
    if (amount == 0) {
      return Collections.singleton(purse);
    } else {
      final Set<Purse> purses = new HashSet<Purse>();
      
      for (Coin coin : Coin.values()) {
        if (coin.value > amount) {
          break;
        }
        
        purses.addAll(getPurses(new Purse(purse).putInto(coin), 0));
        purses.addAll(getPurses(purse, amount - coin.value));
      }
      
      memo.put(amount, purses);
      
      return purses;
    }
  }
                    
  @Test
  public void eyeballTest() {
    System.out.println(getPurses(new Purse(), 200).size());
  }
  
  public static void main(String[] args) {
    JUnitCore.main("Solution");
  }
}