LeetCode刷题实战273：整数转换英文表示-轻识

算法的重要性，我就不多说了吧，想去大厂，就必须要经过基础知识和业务逻辑面试+算法面试。所以，为了提高大家的算法能力，这个公众号后续每天带大家做一道算法题，题目就从LeetCode上面选！

今天和大家聊的问题叫做 整数转换英文表示，我们先来看题面：

https://leetcode-cn.com/problems/integer-to-english-words/

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

将非负整数 num 转换为其对应的英文表示。

示例

示例 1：

输入：num = 123
输出："One Hundred Twenty Three"

示例 2：

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

示例 3：

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

示例 4：

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

解题

这个是网上比较好理解的了。

class Solution {
    private final String[] THOUSAND = {"", "Thousand", "Million", "Billion"};
    private final String[] LESS_THAN_TWENTY = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    private final String[] HUNDRED = {"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
    public String numberToWords(int num) {
        if(num == 0) return "Zero";

        StringBuilder sb = new StringBuilder();
        int index = 0;
        while(num > 0) {
            if(num % 1000 != 0) {
                StringBuilder tmp = new StringBuilder();
                helper(num % 1000, tmp);
                sb.insert(0, tmp.append(THOUSAND[index]).append(" "));
            }
            index++;
            num /= 1000;
        }
        return sb.toString().trim();
    }

    private void helper(int num, StringBuilder tmp) {
        if(num == 0) return;
        if(num < 20) {
            tmp.append(LESS_THAN_TWENTY[num]).append(" ");
        }else if(num < 100) {
            tmp.append(HUNDRED[num / 10]).append(" ");
            helper(num % 10, tmp);
        }else {
            tmp.append(LESS_THAN_TWENTY[num / 100]).append(" Hundred").append(" ");
            helper(num % 100, tmp);
        }
    }
}