题目：Integer to English Words

Convert a non-negative integer to its english words representation. Given input is guaranteed to be less than 231 - 1.

For example,

123 -> "One Hundred Twenty Three"12345 -> "Twelve Thousand Three Hundred Forty Five"1234567 -> "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

将数字转为英文表示。

思路：

主要需要分情况讨论：设数字为n

1.当n < 20时，可以直接使用一个单词表示；

　　vector<string>numsb20 = { "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen" };

2.当20 <= n < 100时，需要几十的表示法，

　　vector<string>numsb100 = { "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety" };

3.当100 <= n < 1000时，需要百的表示法；

4.当1000 <= n < 1000000000时，需要千、百万、十亿的表示法，

　　他们每个间隔1000，其中包括上面的三种情况的表示法，注意int的范围使超过十亿的位数的数值不会超过20；

5.0特殊处理。

string LeetCode::numberToWords(int num){    if (!num)return "Zero";    string str;    vector
     
      numsb20 = { "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen" };    vector
      
       numsb100 = { "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety" };    while (num){        if (num >= 1000000000){
   //十亿            int i = num / 1000000000;//int范围使i最大值不可能超过20            num %= 1000000000;            str += numsb20.at(i - 1) + " Billion ";        }        else if (num >= 1000000){
   //百万            int i = num / 1000000;            num %= 1000000;            if (i >= 100){
   //i超过一百                str += numsb20.at(i / 100 - 1) + " Hundred ";                i %= 100;            }            if (i >= 20){
   //i需要使用20以上的数表示                str += numsb100.at(i / 10 - 2) + " ";                i %= 10;            }            if(i)str += numsb20.at(i - 1) + " Million ";            else str += "Million ";        }        else if (num >= 1000){
   //一千            int i = num / 1000;            num %= 1000;            if (i >= 100){                str += numsb20.at(i / 100 - 1) + " Hundred ";                i %= 100;            }            if (i >= 20){                str += numsb100.at(i / 10 - 2) + " ";                i %= 10;            }            if(i)str += numsb20.at(i - 1) + " Thousand ";            else str += "Thousand ";        }        else if (num >= 100){
   //一百            int i = num / 100;            num %= 100;            str += numsb20.at(i - 1) + " Hundred ";        }        else if (num >= 20){
   //几十            int i = num / 10 - 2;            num %= 10;            str += numsb100.at(i) + " ";        }        else if (num >= 1){
   //个位或十几            str += numsb20.at(num - 1) + " ";            num = 0;        }    }    str.pop_back();    return str;}