String - 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.

Example 1:

Input: 123
Output: "One Hundred Twenty Three"

Example 2:

Input: 12345
Output: "Twelve Thousand Three Hundred Forty Five"

思路：

把数字按照英文输出，这和有一题读阿拉伯数字是类似的，就是一个实现题，思想就是把"Thousand", "Million", "Billion"作为单位，分段计算每一段应该读做多少，然后加上这个单位就可以。

代码：

java：

class Solution {

    private static 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 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 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);
        }
    }
}