Leetcode 0273: 整数转换英文表示 Integer to English Words

题目描述：

Convert a non-negative integer num to its English words representation.

中文描述：

将非负整数 num 转换为其对应的英文表示。

Example 1:

Input: num = 123
Output: “One Hundred Twenty Three”

Example 2:

Input: num = 12345
Output: “Twelve Thousand Three Hundred Forty Five”

Example 3:

Input: num = 1234567
Output: “One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven”

Example 4:

Input: num = 1234567891
Output: “One Billion Two Hundred Thirty Four Million Five Hundred Sixty Seven Thousand Eight Hundred Ninety One”

Constraints:

0 <= num <= 2³¹ - 1

Time complexity: $O$($N$)
分治法：
根据英语表示位数的有 Hundred 百，Thousand 千， Million 百万，Billion十亿。我们可以用分治法将数字拆封成不同的位数然后递归处理。
例如：1234567890 对用的英语 位数表示 1 Billion 234 Million 567 Thousand 890。
在每个最听的位数表示中 每个数做多是三位数。
总结所有情况：

1. ＝０返回"";
2. (0, 20] 用数组存储１～０的英文单词，返回该数字的英文
3. (20, 100] 用数组存储 20 ～ 90 整十的英文，返回十位的英文＋个位的英文
4. (100, 1000] 百位数，返回百位以上数字(递归)(当前数/100)＋百位单位＋百位以下的数(递归)(当前数%100)
5. (1000, 10⁶] 千位数，返回千位以上数字(递归)(当前数/1000)＋千位单位＋千位以下的数(递归)(当前数%1000)
6. (10⁶, 10⁹] 百万位数，返回百万位以上数字(递归)(当前数/10⁶)＋百万位单位＋百万位以下的数(递归)(当前数%10⁶)
7. (10⁹，2³¹ - 1] 十亿位数，返回十亿位以上数字(递归)(当前数/10⁹)＋十亿位单位＋十亿位以下的数(递归)(当前数%10⁹)

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[] TENS = {
    
    "", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
    public String numberToWords(int num) {
    
    
        if (num == 0) return "Zero";
        String res = helper(num);
        return res.trim();
    }

    private String helper(int num) {
    
    
        StringBuilder sb = new StringBuilder();
        if (num < 20) {
    
    
            sb.append(Less20[num]);
        } else if (num < 100) {
    
    
            sb.append(TENS[(num/10)]).append(" ").append(Less20[num % 10]);
        } else if (num < 1000) {
    
    
            sb.append(helper(num / 100)).append(" Hundred ").append(helper(num % 100));
        } else if (num < 1000000) {
    
    
             sb.append(helper(num / 1000)).append(" Thousand ").append(helper(num % 1000));
        } else if (num < 1000000000) {
    
    
             sb.append(helper(num / 1000000)).append(" Million ").append(helper(num %1000000));
        } else {
    
    
             sb.append(helper(num / 1000000000)).append(" Billion ").append(helper(num %1000000000));
        }
        return sb.toString().trim();

    }
}