273. Integer to English Words

273. 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"

思路： 这题的特点就是英语是以1000为单位进行操作的，不像汉语用“万”。 英语的另一个特点就是小于20的数字要特殊对待，整十的数字也要特殊对待。 一个数字num, 分为num<20, num<100提取整十，num<1000提取hundreds,再处理剩下小于100的部分。 大于1000的数字，有thuansands, million, billion. 1000以内的按上一行的方法处理。

class Solution { private static final String[] lessThan20 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen","Fifteen","Sixteen","Seventeen","Eighteen","Nineteen"}; private static final String[] tens = {"","Ten","Twenty","Thirty","Forty","Fifty","Sixty","Seventy","Eighty","Ninety"}; private static final String[] thousands = {"","Thousand","Million","Billion"}; public String numberToWords(int num) { if(num == 0) return "Zero"; String word = ""; int i = 0; while(num > 0){ if(num%1000 != 0){ word = helper(num%1000) + thousands[i] + " " + word; } num = num/1000; i++; } return word.trim(); } public String helper(int n) { if(n == 0){ return ""; } else if(n<20){ return lessThan20[n] + " "; } else if(n<100){ return tens[n/10] + " " + helper(n%10); } else { return lessThan20[n/100] + " Hundred " + helper(n%100); } }}

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