leetcode 273 Integer to English Words

lc273 Integer to English Words

分析英文的计数规则，1000一个循环，thousand，million，billion，trillion，所以我们可以每1000处理一次，后面加上相应的thousand或是million…

然后再看1000以内规则：

hundred一档，ten一档，0~9一档

不过要注意11,12,13~19是特殊的，

所以我们可以先处理>=100的部分，然后是>=20的部分，最后是剩下的<20部分

class Solution {
        private final String[] less20 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
        private final String[] less100 = {"", "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;
            }
            i++;
            num /= 1000;
        }
        return res.trim();
    }
    
    private String helper(int num){
        if(num == 0)
            return "";
        else if(num < 20)
            return less20[num] + " ";
        else if(num < 100)
            return less100[num / 10] + " " + helper(num % 10);
        else
            return less20[num / 100] + " Hundred " + helper(num % 100);
    }
}