题目

给定一个整数，打印该整数的英文描述。

示例 1:

输入: 123
输出: "One Hundred Twenty Three"

示例 2:

输入: 12345
输出: "Twelve Thousand Three Hundred Forty Five"

示例 3:

输入: 1234567
输出: "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

示例 4:

输入: 1234567891
输出: "One Billion Two Hundred Thirty Four Million Five Hundred Sixty Seven Thousand Eight Hundred Ninety One"

注意：本题与 273 题相同：https://leetcode-cn.com/problems/integer-to-english-words/

题解

public String numberToWords(int num) {
    String[] exact = new String[]{
        // 1~19
        "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten",
        "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen",
        // 20~90
        "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"
    };

    // 999及以下的数字转英文
    Function<Integer, List<String>> converter = number -> {
        List<String> list = new ArrayList<>();
        int hundred = number / 100;
        // 超过100的只会是个位1~9
        if (hundred > 0) {
            list.add(exact[hundred - 1]);
            list.add("Hundred");
        }

        int left = number % 100;
        // 数字超过20
        if (left >= 20) {
            list.add(exact[left / 10 + 17]);
            left %= 10;
        }

        // 若未超过20 根据索引直接取对应值 超过20只会是1~9
        if (left > 0) {
            list.add(exact[left - 1]);
        }

        return list;
    };

    List<String> result = new ArrayList<>();

    // 单位切换
    String[] unit = new String[]{null, "Thousand", "Million", "Billion"};
    List<Integer> numbers = new ArrayList<>();
    // 从低到高每3为分为一段
    while (num > 0) {
        numbers.add(num % 1000);
        num /= 1000;
    }

    for (int i = numbers.size() - 1; i >= 0; i--) {
        int section = numbers.get(i);
        // 若段的数字为0 不转换
        if (section > 0) {
            result.addAll(converter.apply(numbers.get(i)));
            // 最低段没有单位
            if (i > 0) {
                result.add(unit[i]);
            }
        }
    }

    // 只有0
    if (result.isEmpty()) {
        result.add("Zero");
    }

    return String.join(" ", result);
}