The task “Integer to English Words” can be solved as follows:

- introduce arrays for big numbers (100, 1000, 1 000 000 and so on) and for numbers less than 20
- introduce arrays for names of big numbers and for names of numbers less than 20
- use a recursive approach:

– if the current value ot the num is greater than the smallest number from the big numbers array:

— find the highest divider of the current*num*value from an array of big numbers

— append a result string with the recursive result of the current function with*num*as a reminder from division a*num*by the highest divider

— append a result string with the current English name of divider

— subtract the divider from the*num*

– otherwise find the highest divider of the current value of the*num*from array of small numbers

— append a result string with the current English name of divider

— subtract the divider from the*num*

— append a result string with the recursive result of the current function with*num*as a reminder from division a num by the highest divider

– recursively call the current function with a*num*as the reminder from division the*num*by the highest divider - time complexity:
*O*(n), where n – count of digits in the num - space complexity:
*O*(1)

```
public class IntegerToEnglishWords {
private final String[] symbols = new String[]{"One", "Two", "Three", "Four", "Five", "Six", "Seven",
"Eight",
"Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen",
"Seventeen", "Eighteen", "Nineteen", "Twenty", "Thirty", "Forty", "Fifty", "Sixty",
"Seventy",
"Eighty", "Ninety"};
private final short[] numbers = new short[]{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,
18, 19,
20, 30, 40, 50, 60, 70, 80, 90};
private final String[] bigSymbols = new String[]{"Hundred", "Thousand", "Million", "Billion"};
private final int[] bigNumbers = new int[]{100, 1000, 1000000, 1000000000};
public String numberToWords(int num) {
if (num == 0) {
return "Zero";
}
return numberToWordsRecursive(num);
}
private String numberToWordsRecursive(int num) {
if (num == 0) {
return "";
}
var resultLeft = new StringBuilder();
if (num >= bigNumbers[0]) {
var j = bigNumbers.length - 1;
while (num / bigNumbers[j] == 0) {
j--;
}
resultLeft.append(numberToWordsRecursive(num / bigNumbers[j]));
resultLeft.append(" ");
resultLeft.append(bigSymbols[j]);
resultLeft.append(" ");
num -= bigNumbers[j] * (num / bigNumbers[j]);
} else {
var j = numbers.length - 1;
while (num / numbers[j] == 0) {
j--;
}
resultLeft.append(symbols[j]);
num -= numbers[j] * (num / numbers[j]);
resultLeft.append(numberToWordsRecursive(num / numbers[j]));
resultLeft.append(" ");
}
resultLeft.append(numberToWordsRecursive(num));
return resultLeft.toString().trim();
}
public static void main(String[] args) {
System.out.println(new IntegerToEnglishWords().numberToWords(1));
}
}
```