一、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”

  点击此处跳转题目。

二、C# 题解

  由于英文数字表示的特点，将整数每隔 3 位分为一组，从右到左分别对应 “Zero”, “Thousand”, “Million”, “Billion”。因此，现在只需要求解 1 ~ 999 的英文表示，记为 sw（SmallWord）。

• 1 - 20：特殊部分，无规律。
• 100 以内整十：特殊部分，无规律。
• 非整十二位数：整十部分 + 个位数。
• 100 - 999：整百部分 + 二位数。

  因此，所有整数可以表示为：
$$sw(Billion)|sw(Million)|sw(Thousand)|sw$$

  如果 sw 为 “”，其对应括号内的内容不显示。

public class Solution {private static string[] Words0_9 = {"Zero", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine",};private static string[] Words10_19 = {"Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen",};private static string[] Words_x0 = { // 整十数"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety",};private static string[] Words_Huge = {"", "Thousand", "Million", "Billion",};private static string Word_100 = "Hundred";public string NumberToWords(int num) {if (num == 0) return Words0_9[0];int           cnt = 0;StringBuilder ans = new StringBuilder();while (num != 0) {StringBuilder sw = SmallNumberToWords(num % 1000); // 计算当前组的英文 swif (sw.Length != 0)                                // sw 不为空，则与之前的结果进行拼接ans = Union(sw, new StringBuilder(Words_Huge[cnt]), ans);num /= 1000; // num 进入下一组cnt++;}return ans.ToString();}// 返回 1 ~ 999 的英文单词public StringBuilder SmallNumberToWords(int num) {return num switch {>= 1000 => new StringBuilder("Wrong"),>= 100  => Union(new StringBuilder(Words0_9[num / 100]), new StringBuilder(Word_100), SmallNumberToWords(num % 100)),>= 20   => Union(new StringBuilder(Words_x0[num / 10]), SmallNumberToWords(num % 10)),>= 10   => new StringBuilder(Words10_19[num - 10]),_       => new StringBuilder(num > 0 ? Words0_9[num] : "")};}// 拼接字符串private StringBuilder Union(params StringBuilder[] str) {StringBuilder ans = str[0];for (int i = 1; i < str.Length; i++) {if (str[i].Length == 0) continue;     // 如果字符串为空，则直接跳过if (ans.Length != 0) ans.Append(' '); // 已有的字符不为空，则需要在中间加入一个空格ans.Append(str[i]);}return ans;}
}

• 时间：60 ms，击败 100.00% 使用 C# 的用户
• 内存：36.24 MB，击败 100.00% 使用 C# 的用户