LuhnAlgorithm.java

// Luhn Algorithm
// The Luhn algorithm, also known as the mod 10 algorithm, is a simple checksum formula used to validate various identification numbers,
//  such as credit card numbers, IMEI numbers etc. It is mostly used as a simple method to distinguish valid numbers from mistyped or 
// otherwise incorrect numbers. 


// Approach and Explanation

// 1. Start from the rightmost digit (excluding any check digit), double the value of every second digit. 

// 2. If doubling of a number results in a two digit number , greater than 9 then add the digits of the product.

// 3. Sum up all the digits, including the modified ones from the previous step, and calculate the total.

// 4. If the total is divisible by 10 (i.e., the modulo 10 operation results in 0), the number is considered valid according to the Luhn algorithm.

// Time Complexity->	O(n)
// Space Complexity->	O(1)

// Implementation

import java.util.Scanner;
// function to check validity of a number
public class LuhnAlgorithm {
    public static boolean luhnAlgorithm(String number) {
        int n = number.length();
        int sum = 0;
        boolean isSecondDigit = false;

        // Traverse the number from right to left
        for (int i = n - 1; i >= 0; i--) {
            int digit = number.charAt(i) - '0';

            // Double every second digit, starting from the rightmost
            if (isSecondDigit) {
                digit *= 2;
            }

            // Adding the 2 digit to sum if the digits produced is double digit
            sum += digit / 10; // If the no. is <9 then this step will produce 0
            sum += digit % 10;

            // Toggle the flag for the next iteration
            isSecondDigit = !isSecondDigit;
        }

        // If the sum is divisible by 10, the number is valid
        return (sum % 10 == 0);
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter the number: ");
        String number = scanner.nextLine();

        // Check if the number is valid using the Luhn algorithm
        if (luhnAlgorithm(number)) {
            System.out.println("Valid number.");
        } else {
            System.out.println("Invalid number.");
        }
        scanner.close();
    }
}