题目描述：
Convert a non-negative integer to its english words representation. Given input is guaranteed to be less than 231 - 1.
For example,
123 -> "One Hundred Twenty Three"
12345 -> "Twelve Thousand Three Hundred Forty Five"
1234567 -> "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"
就是把数字转化为英语单词。
思路：
1.先把从100-999映射为英文单词的函数写出来：
1.1先考虑一位数：一一映射就可以了。函数为Digit()
1.2然后考虑两位数的情况：需要区分11-19，函数为Teen()和20-99的情况，函数为Ty()
1.3然后对百位数调用Digit()就可以了。如果百位数为0，则调用Teen()或Ty()
函数存为Hundred()
2.考虑小于等于6位数的情况，即百万以下的数。
将这六位数分为前3位和后3位，分别调用Hundred()函数，中间插入"Thousand"即可。
函数存为Thousand()
3.考虑小于等于9位数的情况，同6位数处理过程类似，考虑前3位和后六位，分别调用Thousand()，中间插入"Million"即可
函数存为Million()
4.考虑9位数以上的情况，同理，考虑前3位和后9位，分别调用Million()函数，中间插入"Billion"
5.需要注意的是，所有输出都是首字母大写
本题的难点在于情况特别多，边界值的情况尤其要注意。问题分类的话算是数学问题。
实现代码：public class Solution {
public string NumberToWords(int num)
{
if(num == 0){
return "Zero";
}
var str = num.ToString();
if(num > 999999999){
var s1 = str.Substring(0,str.Length - 9);
var n1 = int.Parse(s1);
var s2 = str.Substring(str.Length - 9, 9);
var s = Millions(s1) + " Billion " + Millions(s2);
s = s.TrimEnd();
return s;
}
else{
return Millions(str);
}
}
private string Millions(string str)
{
var n = int.Parse(str);
if(n > 999999){
var s1 = str.Substring(0,str.Length - 6);
var n1 = int.Parse(s1);
var s2 = str.Substring(str.Length - 6, 6);
var s = Thousands(s1) + " Million " + Thousands(s2);
s = s.TrimEnd();
return s;
}
else{
return Thousands(n.ToString());
}
}
private string Thousands(string str)
{
var n = int.Parse(str);
if(n > 999){
var s1 = str.Substring(0,str.Length - 3);
var n1 = int.Parse(s1);
var s2 = str.Substring(str.Length - 3, 3);
var s = Hundreds(s1)+ " Thousand " + Hundreds(s2);
s = s.TrimEnd();
return s;
}
else{
return Hundreds(n.ToString());
}
}
private string Hundreds(string str)
{
int n = int.Parse(str);
if(n < 10){
return Digit(str.Last());
}
else if(n == 10){
return "Ten";
}
else if(n > 10 && n < 20)
{
return Teen(str.Substring(str.Length - 2,2));
}
else if(n >= 20 && n < 100)
{
return Ty(str.Substring(str.Length - 2,2));
}
else if(n >= 100 && n <= 999)
{
var c1 = str[0];
var s = Digit(c1) + " Hundred";
var c2 = str[1];
var c3 = str[2];
if(c2 == '0'){
return c3 != '0' ? s + " " + Digit(c3) : s;
}
if(c2 == '1'){
return s + " " + Teen(str.Substring(str.Length - 2,2));
}
else{
return s + " " + Ty(str.Substring(str.Length - 2,2));
}
}
else{
throw new ArgumentException("input should be less than 1000");
}
}
private string Teen(string str)
{
switch(str){
case "10":
return "Ten";
case "11":
return "Eleven";
case "12":
return "Twelve";
case "13":
return "Thirteen";
case "14":
return "Fourteen";
case "15":
return "Fifteen";
case "16":
return "Sixteen";
case "17":
return "Seventeen";
case "18":
return "Eighteen";
case "19":
return "Nineteen";
default :
throw new ArgumentException("input should be from 11 to 19 ONLY.");
}
}
private string Ty(string str)
{
var c1 = str[0];
var c2 = str[1];
var space = c2 == '0' ? "" : " ";
switch(c1){
case '2':
return "Twenty" +space+ Digit(c2);
case '3':
return "Thirty" +space+ Digit(c2);
case '4':
return "Forty" +space+ Digit(c2);
case '5':
return "Fifty" +space+ Digit(c2);
case '6':
return "Sixty" +space+ Digit(c2);
case '7':
return "Seventy" +space+ Digit(c2);
case '8':
return "Eighty" +space+ Digit(c2);
case '9':
return "Ninety" +space+ Digit(c2);
default :
throw new ArgumentException(string.Format("input should between [20,99], but got {0}",c1));
}
}
private string Digit(char c)
{
switch(c){
case '0':
return "";
case '1':
return "One";
case '2':
return "Two";
case '3':
return "Three";
case '4':
return "Four";
case '5':
return "Five";
case '6':
return "Six";
case '7':
return "Seven";
case '8':
return "Eight";
case '9':
return "Nine";
default :
throw new ArgumentException("input should beteen [0-9]");
}
}
}