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]);
}
}
}
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