leetcode 273. 整数转换英文表示 困难

273. 整数转换英文表示

这道题并不难，但是特别麻烦

我写的代码

class Solution {
public:
    //转换个位数的英文
    string baseNumber(int num){
        if(num == 1)  return "One";
        else if(num == 2)  return "Two";
        else if(num == 3)  return "Three";
        else if(num == 4)  return "Four";
        else if(num == 5)  return "Five";
        else if(num == 6)  return "Six";
        else if(num == 7)  return "Seven";
        else if(num == 8)  return "Eight";
        else if(num == 9)  return "Nine";
        else  return "";  //个位是0，返回空串
    }
    //转换两位数的英文
    string tenNumber(int num){
        if(num == 0)  return "";  //两位数都是0，返回空串
        if(num >= 10 && num <= 19){
            if(num == 10)  return "Ten";
            else if(num == 11)  return "Eleven";
            else if(num == 12)  return "Twelve";
            else if(num == 13)  return "Thirteen";
            else if(num == 14)  return "Fourteen";
            else if(num == 15)  return "Fifteen";
            else if(num == 16)  return "Sixteen";
            else if(num == 17)  return "Seventeen";
            else if(num == 18)  return "Eighteen";
            else if(num == 19)  return "Nineteen";
        }
        else if(num <= 9)  return baseNumber(num);
        //接下来的是20-99的转换
        int ten = num/10;
        int base = num%10;
        string res = "";
        //twenty thirty fourty fifty sixty seventy eighty ninety
        if(ten == 2)        res+="Twenty";
        else if(ten == 3)  res+="Thirty";
        else if(ten == 4)  res+="Forty";
        else if(ten == 5)  res+="Fifty";
        else if(ten == 6)  res+="Sixty";
        else if(ten == 7)  res+="Seventy";
        else if(ten == 8)  res+="Eighty";
        else if(ten == 9)  res+="Ninety";
        if(base == 0)  return res;
        return res + " " + baseNumber(base);
    }
    //转换三位数的英文
    string hundredNumber(int num){
        if(num == 0)  return ""; //三位数都是0，返回空串
        if(num < 100) return tenNumber(num);
        if(num % 100 == 0) return baseNumber(num/100) + " " + "Hundred";
        return baseNumber(num / 100) + " " + "Hundred" + " " + tenNumber(num % 100);  
    }
    string numberToWords(int num) {
        if(num == 0) return "Zero";
        string res="";

        //hundred 100 thousand 1000 million 1000000 billion 1000000000
        int billion = num / 1000000000;
        if(billion != 0)  res = res + baseNumber(billion) + " " + "Billion" + " ";
        num = num % 1000000000;

        int million = num / 1000000;
        if(million != 0)  res = res + hundredNumber(million) + " " + "Million" + " ";
        num = num % 1000000;

        int thousand = num / 1000;
        if(thousand != 0)  res = res + hundredNumber(thousand) + " " + "Thousand" + " ";
        num = num % 1000;
        
        res = res + hundredNumber(num);
        if(res[res.size()-1] == ' ')  res.pop_back();//如果最后有多余的空格就去掉

        return res;
    }
};