题解 | #分治学英语#

import java.util.Scanner;
/***
*** 利用每三位一组的特性，在数字表达上从高位到低位(字符串从左到右)的顺序开始学，在整体上按照""、"thousand"、"million"、"billion"每三位进行分配，分配的原则是碰到组中的最低位(0)进行分配。
注意点: 因为字符从左到右是0到len-1的顺序，但是在数字中要转换一下对应位数分组位:(str.length() - 1 - i) / 3，
(str.length() - 1 - i) % 3每组中俄位数。剩下就是里面的分支判断了，感觉这样写下来比较清晰，好理解。
***/
// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    static String[] spk_bmt = {"", "thousand", "million", "billion"};
    static String[] spk_nums_ge = {"", "one", "two", "three", "four", "five", "six",
                                   "seven", "eight", "nine"
                                  };
    static String[] spk_nums_shi = {"", "ten", "twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety"};
    static String[] spk_nums_one_shi = {"ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"};

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        while (in.hasNextLong()) { // 注意 while 处理多个 case
            Long num = in.nextLong();
            System.out.println(speakNum(num));
        }
    }

    public static String speakNum(Long num) {
        String spk_ans = "";
        String str = String.valueOf(num);
        int len = str.length();
        if (len == 1) {
            return spk_nums_ge[Integer.valueOf(str)];
        }

        for (int i = 0; i < str.length(); i++) {
            char c = str.charAt(i);
            int bit_num = Character.getNumericValue(c);

            String speak_num = "";
            int part = (str.length() - 1 - i) / 3; 
            if ((str.length() - 1 - i) % 3 == 0 ) { // 个位
                if (i - 2 >= 0 && str.charAt(i - 1) == '0' && str.charAt(i - 2) == '0' &&
                        str.charAt(i) == '0') {
                    continue;
                }
                speak_num = spk_nums_ge[bit_num];
                speak_num = speak_num + " " + spk_bmt[part] + " ";

            } else if ((str.length() - 1 - i) % 3 == 2) { // 百位
                speak_num = spk_nums_ge[bit_num];
                if (str.charAt(i) != '0') {
                    speak_num = speak_num + " hundred ";
                    if (str.charAt(i + 1) != '0' || str.charAt(i + 2) != '0') {
                        speak_num = speak_num + "and ";
                    }
                }
            } else if ((str.length() - 1 - i) % 3 == 1) { //十位
                if (str.charAt(i) == '1') {
                    speak_num = spk_nums_one_shi[Character.getNumericValue(str.charAt(i + 1))];
                    i++;
                } else {
                    speak_num = spk_nums_shi[bit_num] + " ";
                }
            }
            spk_ans = spk_ans + speak_num;
        }
        return spk_ans.replaceAll("  ", " ");
    }
}