Learn how to Discover the nth Reverse Quantity in Java

The problem

Reverse Quantity is a quantity which is similar when reversed.

For instance, the primary 20 Reverse Numbers are:

0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 22, 33, 44, 55, 66, 77, 88, 99, 101

Activity

• You might want to return the nth reverse quantity. (Assume that reverse numbers begin from 0 as proven within the instance.)

Notes

The answer in Java

Possibility 1:

import java.math.BigInteger;

public class Palindrome {

    personal static int size = 0;

    public static BigInteger findReverseNumber(lengthy n) {
        if (n == 1) {
            return BigInteger.ZERO;
        }
        size = 0;
        String the rest = findLengthAndRemainder(n - 1);
        whereas (the rest.size() < (size + 1) / 2) {
            the rest = "0" + the rest;
        }
        StringBuilder end result = new StringBuilder();
        if (the rest.matches("d0*")) {
            end result.append(the rest.charAt(0));
            end result.append("9".repeat(the rest.size() - 1));
        } else {
            end result.append(Integer.parseInt(the rest.substring(0, 1)) + 1);
            String worth = String.valueOf(new BigInteger(the rest.substring(1)).subtract(BigInteger.ONE));
            whereas (worth.size() < the rest.size() - 1) {
                worth = "0" + worth;
            }
            end result.append(worth);
        }
        String firstHalf = end result.toString();
        String secondHalf = end result.reverse().substring(size % 2);
        return new BigInteger(firstHalf + secondHalf);
    }

    personal static String findLengthAndRemainder(lengthy quantity) {
        lengthy subtract = 9;
        whereas (quantity > 0) {
            size++;
            quantity -= subtract;
            if (quantity <= 0) {
                break;
            }
            size++;
            quantity -= subtract;
            if (quantity > 0) {
                subtract *= 10;
            }
        }
        quantity += subtract;
        return String.valueOf(quantity);
    }

}

Check instances to validate our answer

import org.junit.jupiter.api.Check;

import java.math.BigInteger;

import static org.junit.jupiter.api.Assertions.assertEquals;

class PalindromeTest {

    @Check
    void testFixed() {
        assertEquals(new BigInteger("0"), Palindrome.findReverseNumber(1));
        assertEquals(new BigInteger("1"), Palindrome.findReverseNumber(2));
        assertEquals(new BigInteger("9"), Palindrome.findReverseNumber(10));
        assertEquals(new BigInteger("909"), Palindrome.findReverseNumber(100));
        assertEquals(new BigInteger("900000000000000000009"), Palindrome.findReverseNumber(100000000000L));
    }

}