数字发音

• 有一个非负整数，请编写一个算法，打印该整数的英文描述。
  给定一个int x，请返回一个string，为该整数的英文描述。
• 测试样例： 1234 返回：”One Thousand,Two Hundred Thirty Four”

private  string[] belowTen = new String[] {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine"}; 
private  string[] belowTwenty = new String[] {"Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"}; 
private  string[] belowHundred = new String[] {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"}; 

public string toString(int x)
{
    if (x == 0) return "Zero"; 
    return helper(x);  
} 

private String helper(int num) { 
    string result = "";
    if (num < 10) result = belowTen[num];
    else if (num < 20) result = belowTwenty[num - 10];
    else if (num < 100) result = belowHundred[num / 10] + " " + helper(num % 10);
    else if (num < 1000) result = helper(num / 100) + " Hundred " + helper(num % 100);
    else if (num < 1000000) result = helper(num / 1000) + " Thousand" +( num % 1000 > 0 ? "," : " ") + helper(num % 1000);
    else if (num < 1000000000) result = helper(num / 1000000) + " Million" + (num % 1000000 > 0 ? "," : " ") + helper(num % 1000000);
    else result = helper(num / 1000000000) + " Billion " + helper(num % 1000000000);
    return result.Trim(' ');
}