题目：

题解：

class Solution {
public:vector<string> singles = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine"};vector<string> teens = {"Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};vector<string> tens = {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};vector<string> thousands = {"", "Thousand", "Million", "Billion"};string numberToWords(int num) {if (num == 0) {return "Zero";}string sb;for (int i = 3, unit = 1000000000; i >= 0; i--, unit /= 1000) {int curNum = num / unit;if (curNum != 0) {num -= curNum * unit;sb = sb + toEnglish(curNum) + thousands[i] + " ";}}while (sb.back() == ' ') {sb.pop_back();}return sb;}string toEnglish(int num) {string curr;int hundred = num / 100;num %= 100;if (hundred != 0) {curr = curr + singles[hundred] + " Hundred ";}int ten = num / 10;if (ten >= 2) {curr = curr + tens[ten] + " ";num %= 10;}if (num > 0 && num < 10) {curr = curr + singles[num] + " ";} else if (num >= 10) {curr = curr + teens[num - 10] + " ";}return curr;}
};