题解 | #学英语#

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        //1≤n≤2000000 其实int类型就够了
        while (in.hasNextLong()){
            long num = in.nextLong();
            invoke(num);
        }
    }

    private static void invoke(long num){
        //定义千分位分隔模式对应的单位
        String[] units = {null,"thousand","million","billion"};

        //把输入的数字转成千分位分隔模式
        String numstr = String.valueOf(num);
        int size = numstr.length() / 3;
        int offset = numstr.length() % 3;
        if(offset > 0){
            size++;
        }

        List<String> res = new LinkedList<>();
        for(int i=0;i<size;i++){
            int begin =0,end=0;
            if(offset == 0){//输入数字正好是3的整数倍
                begin = i*3;
                end = i*3 + 3;
            }else{
                if(i > 0){
                    begin = offset  + (i-1)*3;
                }
                end = offset + i*3;
            }

            int n = Integer.valueOf(numstr.substring(begin,end));
            res.addAll(trans(n));
            res.add(units[size-i-1]);
        }

        boolean first = true;
        for(String s : res){
            if(s == null){//处理占位符，直接跳过当不存在即可
                continue;
            }
            if(!first){
                System.out.print(" ");
            }
            System.out.print(s);
            first = false;
        }

    }

    /**
     * 1-19
     */
    private static String[] numWords1_19 = {null,"one","two","three","four","five","six","seven","eight","nine",
    "ten","eleven","twelve","thirteen","fourteen","fifteen","sixteen","seventeen","eighteen","nineteen"};
    /**
     * 20-90
     */
    private static String[] numWords20_90 = {"","","twenty","thirty","forty","fifty","sixty","seventy","eighty","ninety"};
    /**
     * 把三位以内的数字转成英文
     * @param num
     * @return 为方便给单词加间隔，先暂存结果的所有单词再格式化打印
     */
    private static List<String> trans(int num){
        List<String> res = new ArrayList<>();
        int hundred = num/100;
        if(hundred > 0){
            res.add(numWords1_19[hundred]);
            res.add("hundred");
        }

        int other = num % 100;
        if(other == 0){
           return res;
        }

        if(res.size() > 0){
            res.add("and");
        }
        if(other >= 20){
            int ten = other / 10;
            res.add(numWords20_90[ten]);
            int one = other % 10;
            res.add(numWords1_19[one]);
        }else{
            res.add(numWords1_19[other]);
        }
        return res;
    }


}