将非负整数转换为其对应的英文表示，给定的输入是保证小于 231 - 1 的。
示例：
123 -> "One Hundred Twenty Three"
12345 -> "Twelve Thousand Three Hundred Forty Five"
1234567 -> "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"
详见：https://leetcode.com/problems/integer-to-english-words/description/
class Solution {
public:
string numberToWords(int num) {
string res = convertHundred(num % 1000);
vector<string> v = {"Thousand", "Million", "Billion"};
for (int i = 0; i < 3; ++i)
{
num /= 1000;
res = num % 1000 ? convertHundred(num % 1000) + " " + v[i] + " " + res : res;
}
while (res.back() == ‘ ‘)
{
res.pop_back();
}
return res.empty() ? "Zero" : res;
}
string convertHundred(int num) {
vector<string> v1 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
vector<string> v2 = {"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
string res;
int a = num / 100, b = num % 100, c = num % 10;
res = b < 20 ? v1[b] : v2[b / 10] + (c ? " " + v1[c] : "");
if (a > 0)
{
res = v1[a] + " Hundred" + (b ? " " + res : "");
}
return res;
}
};
参考：https://www.cnblogs.com/grandyang/p/4772780.html
273 Integer to English Words 整数转换英文表示
原文：https://www.cnblogs.com/xidian2014/p/8761300.html