https://leetcode.com/problems/integer-to-english-words/?tab=Description
将数字转成英文读音
求1000以内用helper函数，倒序，先“Thousand”再“Million”
public class Solution { private final String[] LESS_THAN_20 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"}; private final String[] TENS = {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"}; private final String[] THOUSANDS = {"", "Thousand", "Million", "Billion"}; public String numberToWords(int num) { if (num == 0) { return "Zero"; } String res = ""; int i = 0; while (num != 0) { if (num % 1000 != 0) { res = helper(num % 1000) + THOUSANDS[i] + " " + res; } num /= 1000; i++; } return res.trim(); } private String helper(int num) { if (num == 0) { return ""; } else if (num < 20) { return LESS_THAN_20[num] + " "; } else if (num < 100) { return TENS[num / 10] + " " + helper(num % 10); } else { return LESS_THAN_20[num / 100] + " " + "Hundred" + " " + helper(num % 100); } } }