Project Euler > Problem 17 > Number letter counts (Java Solution)

Problem:

If the numbers 1 to 5 are written out in words: one, two, three, four, five, then there are 3 + 3 + 5 + 4 + 4 = 19 letters used in total.

If all the numbers from 1 to 1000 (one thousand) inclusive were written out in words, how many letters would be used?


NOTE: Do not count spaces or hyphens. For example, 342 (three hundred and forty-two) contains 23 letters and 115 (one hundred and fifteen) contains 20 letters. The use of "and" when writing out numbers is in compliance with British usage.

Solution:

21124

Code:
The solution may include methods that will be found here: Library.java .

public interface EulerSolution{

public String run();

}

/* 
 * Solution to Project Euler problem 17
 * By Nayuki Minase
 * 
 * http://nayuki.eigenstate.org/page/project-euler-solutions
 * https://github.com/nayuki/Project-Euler-solutions
 */


public final class p017 implements EulerSolution {
 
 public static void main(String[] args) {
  System.out.println(new p017().run());
 }
 
 
 public String run() {
  int sum = 0;
  for (int i = 1; i <= 1000; i++)
   sum += toEnglish(i).length();
  return Integer.toString(sum);
 }
 
 
 private static String[] ONES = {"zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"};  // Requires 0 <= n <= 9
 private static String[] TEENS = {"ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"};
 private static String[] TENS = {"twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety"};
 
 
 // Requires 0 <= n <= 99999
 private static String toEnglish(int n) {
  if (n < 0 || n > 99999)
   throw new IllegalArgumentException();
  
  if (n < 100)
   return tens(n);
  else {
   String big = "";
   if (n >= 1000)
    big += tens(n / 1000) + "thousand";
   if (n / 100 % 10 != 0)
    big += ONES[n / 100 % 10] + "hundred";
   
   return big + (n % 100 != 0 ? "and" + tens(n % 100) : "");
  }
 }
 
 
 // Requires 0 <= n <= 99
 private static String tens(int n) {
  if (n < 10)
   return ONES[n];
  else if (n < 20)  // Teens
   return TEENS[n - 10];
  else
   return TENS[n / 10 - 2] + (n % 10 != 0 ? ONES[n % 10] : "");
 }
 
}