[LeetCode]Integer to English Words

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