Luhn Check (Card Number Validation)

/*
 * Problem : Process one line of text.
 * Input   : One line of text.
 * Output  : true or false.
 * Approach: Walk from the right.
 * Time    : O(n)
 * Space   : O(1)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    // This method solves the task. It receives the input and returns the answer.
    static boolean solve(String s) {
        // Step 1: The sum starts at 0.
        int sum = 0;

        // Step 2: Go from the last digit to the first digit. position 0 is the last digit.
        for (int i = s.length() - 1, position = 0; i >= 0; i--, position++) {
            int digit = s.charAt(i) - '0';

            // Step 3: Double every second digit. If the result is above 9, subtract 9.
            if (position % 2 == 1) {
                digit = digit * 2;
                if (digit > 9) {
                    digit = digit - 9;
                }
            }

            // Step 4: Add the digit to the sum.
            sum = sum + digit;
        }

        // Step 5: The number is valid when the sum is a multiple of 10.
        return sum % 10 == 0;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String s = scanner.hasNextLine() ? scanner.nextLine() : "";
        System.out.println(solve(s));
    }
}