Cracking Coding Interview - 16.8 English Int

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1: one, two, three, four, five, six, seven, eight, nine
10: ten, twenty, thirty, fourty, fifty, sixty, seventy, eighty, ninety
100: one hundred, two hundred, three hundred, ...
1,000: one thousand, two thousand, three thousand, ...
1,000,000: one million, two million, three million, ...
1,000,000,000: one billion, two billion, three billion, ...

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11      12      13        14        15       16       17         18        19
eleven, twelve, thirteen, fourteen, fifteen, sixteen, seventeen, eighteen, nineteen

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10   20      30      40      50     60     70       80      90
ten, twenty, thirty, fourty, fifty, sixty, seventy, eighty, ninety

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private static final int BILLION = 1000000000;
private static final int MILLION = 1000000;
private static final int THOUSAND = 1000;
private static final int HUNDRED = 100;
private static final int TEN = 10;


private static final String[] TENS = new String {"Twenty", "Thirty", ...};
private static final String[] TEENS = new String {"Ten", "Eleven", ...};
private static final String[] ONES = new String {"One", "Two", ...}

public String toEnglish(int n) {
  String result = "";
  if (n < 0) {
    result += "Negative ";
    n = -n;
  }
  if (n >= BILLION) {
    int c = n / BILLION;
    result += toEnglish(c) + " Billion ";
    n = n % BILLION;
  }
  
  if (n >= MILLION) {
    int c = n / MILLION;
    result += toEnglish(c) + " Million ";
    n = n % MILLION;
  }
  
  if (n >= THOUSAND) {
    int c = n / THOUSAND;
    result += toEnglish(c) + " Thousand ";
    n = n % THOUSAND;
  }
  
  if (n >= HUNDRED) {
    int c = n / HUNDRED;
    result += toEnglishOne(c) + " Hundred ";
    n = n % HUNDRED;
  }
  
  if (n >= TEN) {
    int c = n / TEN;
    if (c > 1) {
      result += toEnglishTen(c);
      n = n % 10;
    } else if (c == 1) {
      result += toEnglishTeen(n);
      return result;
    }
  }
  
  if (n > 0) {
    result += toEnglishOne(n);
  }
  
  return result;
}

public String toEnglishTen(int n) {
  return TENS[n - 2];
}

public String toEnglishTeen(int n) {
  return TEENS[n - 10];
}

public String toEnglishOne(int n) {
  return ONES[n - 1];
}