Question

A palindromic number reads the same both ways (left-to-right and right-to-left).

The largest palindrome made from the product of two 2-digit numbers is 9,009 = 91 × 99.

The largest palindrome made from the product of two 3-digit numbers is 906,609 = 913 × 993.
The largest palindrome made from the product of two 4-digit numbers is 99,000,099 = 9,901 × 9,999.

1. Write a function IN JAVASCRIPT to find the largest palindrome made from the product of two 7-digit numbers.

2. What is that product?

3. How long (execution time) does it take your solution to calculate this answer?

Answer 1

ANSWER :

Code

public class Main {

    private int largestPalindrome(int n) {
        long startTime = System.nanoTime();
        int upperBound = (int) Math.pow(10, n) - 1;
        int lowerBound = (int) Math.pow(10, n - 1);
        int maximum = Integer.MIN_VALUE;
//        int digit1 = 0, digit2 = 0;
        for (int upperIndex = upperBound; upperIndex >= lowerBound; upperIndex--) {
            for (int lowerIndex = upperIndex; lowerIndex >= lowerBound; lowerIndex--) {
                int product = upperIndex * lowerIndex;
                if (product < maximum) {
                    break;
                }
                if (product > maximum && check(product)) {
//                    digit1 = upperIndex;
//                    digit2 = lowerIndex;
                    maximum = product;
                }
            }
        }
        long endTime = System.nanoTime();
        System.out.println("Execution time in milliseconds : " + (endTime - startTime) / 1000000);
//        System.out.println("Digit1 = " + digit1 + " Digit2 = " + digit2);
        return maximum;
    }

    private boolean check(int number) {
        int reverse = 0, curr = number;
        while (number != 0) {
            reverse = reverse * 10 + number % 10;
            number /= 10;
        }
        return curr - reverse == 0;
    }

    public static void main(String[] args) {
        int n = 7;
        Main main = new Main();
        System.out.print(main.largestPalindrome(n));
    }
}

Output

// Uncomment the commented line to print the both the numbers

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