Reverse a Number

/*
 * Problem : Reverse the digits of an integer.
 * Input   : One integer.
 * Output  : The reversed integer.
 * Approach: Build the result by adding one digit at a time.
 * Time    : O(d)
 * Space   : O(1)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        long number = new Scanner(System.in).nextLong();

        // Step 1: Remember the sign. We work with a positive number and add the sign at the end.
        boolean isNegative = number < 0;
        number = Math.abs(number);

        // Step 2: The result starts at 0. We use long because a reversed number can be larger than int.
        long reversed = 0;

        // Step 3: Repeat until no digits are left.
        while (number > 0) {
            // Step 4: Take the last digit. Example: 1234 % 10 = 4.
            long digit = number % 10;

            // Step 5: Move the old result one place to the left and add the digit. Example: 4 * 10 + 3 = 43.
            reversed = reversed * 10 + digit;

            // Step 6: Remove the last digit. Example: 1234 / 10 = 123.
            number = number / 10;
        }

        // Step 7: Put the minus sign back if the input was negative.
        System.out.println(isNegative ? -reversed : reversed);
    }
}