题目：

题解：

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