題意：給出一下正整數，輸出英文表示

思路：每三個數一組，因為最多有三組，對應的分別為Billion,Million,Thousand。然後針對每組的數輸出

程式碼如下：

public class Solution
{
    private String[] bigNum = {"Billion", "Million", "Thousand","" };
    private String[] num_str = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen",
                            "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};

    private String numberToString(int num)
    {
        StringBuilder sb = new StringBuilder();
        int n = num / 100;
        if (n > 0)
        {
            sb.append(num_str[n]);
            sb.append(" Hundred");
        }

        n = num % 100;
        if (n > 0 && n < 20)
        {
            if (sb.length() != 0) sb.append(" ");
            sb.append(num_str[n]);
        }
        else if (n >= 20)
        {
            if (sb.length() != 0) sb.append(" ");
            sb.append(num_str[19 + n / 10 - 1]);
            n %= 10;
            if (n != 0)
            {
                if (sb.length() != 0) sb.append(" ");
                sb.append(num_str[n]);
            }
        }

        return sb.toString();
    }

    public String numberToWords(int num) {
        List<Integer> ar = new ArrayList<Integer>();
        while (num >= 1000) {
            int tmp = num % 1000;
            ar.add(tmp);
            num /= 1000;
        }

        if (num != 0) ar.add(num);
        //System.out.println(ar);

        int size = ar.size();
        //System.out.println("size:" + size);
        StringBuilder sb = new StringBuilder();
        for (int i = size - 1; i >= 0; i--)
        {
            if (ar.get(i) != 0)
            {
                if (sb.length() != 0) sb.append(" ");
                sb.append(numberToString(ar.get(i)));
                if (!bigNum[3 - i].isEmpty()) sb.append(" ");
                sb.append(bigNum[3 - i]);

            }
        }

        if (sb.length() == 0) sb.append("Zero");

        return sb.toString();
    }
}