有1元，5元，10元，20元，50元，组成100元情况

顺序不同只算一种，算出来是343种：

public class EnumCoin {
    private static final int[] COINS = new int[]{1,5,10,20,50};
    private static final int SUM = 100;
    private static int i=1;
    public static void main(String[] args) throws FileNotFoundException{
        File output  = new File("C:\\1.out");
        System.setOut(new PrintStream(output));
        calc(0, 0, "");
        System.out.println("totally "+ (i-1)+ " solutions.");
    }

    private static void calc(int sum, int cionIdex, String pre) {
        if(SUM==sum){
            System.out.println("case "+(i++)+ ": " +pre);
        }
        for(int i=cionIdex;i            if(SUM - sum >= 0){
                calc(sum+COINS[i],i, pre+" "+ COINS[i]);
            }
        }
    }
}