Concatenate.java

public static String int2String(int n) {
    List<String> digits = new ArrayList<>();
    
    while (n > 0) {
        int digit = n % 10;
        digits.add(digit2String(digit));
        n = n / 10;
    }
    
    String result = "";
    for (int i = digits.size() - 1; i >= 0; i--) {
        result += digits.get(i);
    }
    
    return result;
}

public static String digit2String(int digit) {
    // Chosen implementation here...
}