273 Integer to English Words

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本文介绍了一种将非负整数转换为其英文单词表示的方法，适用于0至2^31-1之间的整数。通过递归分解数字并利用特定规则处理每三位数，实现了精确的英文表述。

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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"

Example 3:

Input: 1234567
Output: "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

题目中给足了提示，首先告诉我们要3个一组的进行处理，而且题目中限定了输入数字范围为0到2³¹ - 1之间，最高只能到billion位，3个一组也只需处理四组即可，那么我们需要些一个处理三个一组数字的函数，我们需要把1到19的英文单词都列出来，放到一个数组里，还要把20,30，... 到90的英文单词列出来放到另一个数组里，然后我们需要用写技巧，比如一个三位数n，百位数表示为n/100，后两位数一起表示为n%100，十位数表示为n%100/10，个位数表示为n%10，然后我们看后两位数是否小于20，小于的话直接从数组中取出单词，如果大于等于20的话，则分别将十位和个位数字的单词从两个数组中取出来。然后再来处理百位上的数字，还要记得加上Hundred。主函数中调用四次这个帮助函数，然后中间要插入"Thousand", "Million", "Billion"到对应的位置，最后check一下末尾是否有空格，把空格都删掉，返回的时候检查下输入是否为0，是的话要返回'Zero'

class Solution {
    public String numberToWords(int num) {
        if (num == 0) {
            return "Zero";
        }
        return helper(num);
    }
    
    private final String[] belowTen = new String[] {"", "One", "Two", "Three", "Four",
                                                    "Five", "Six", "Seven", "Eight", "Nine"};
    private final String[] belowTwenty = new String[] {"Ten", "Eleven", "Twelve", "Thirteen",
                                                       "Fourteen", "Fifteen", "Sixteen", "Seventeen",
                                                      "Eighteen", "Nineteen"};
    private final String[] belowHundred = new String[] {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty",
                                                       "Sixty", "Seventy", "Eighty", "Ninety"};
    private String helper(int num) {
        String s = new String();
        if (num < 10) {
            s = belowTen[num];
        } else if (num < 20) {
            s = belowTwenty[num - 10];
        } else if (num < 100) {
            s = belowHundred[num / 10] + " " + helper(num % 10);
        } else if (num < 1000) {
            s = helper(num / 100) + " Hundred " + helper(num % 100);
        } else if (num < 1000000) {
            s = helper(num / 1000) + " Thousand " + helper(num % 1000);
        } else if (num < 1000000000) {
            s = helper(num / 1000000) + " Million " + helper(num % 1000000);
        } else {
            s = helper(num / 1000000000) + " Billion " + helper(num % 1000000000);
        }
        return s.trim();
    }
}