nextPalindrom.java

package io.github.dvegasa;

import org.junit.platform.commons.annotation.Testable;

import java.math.BigInteger;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println(nextPalindrom(sc.nextLine()));
    }

    public static String nextPalindrom(String number) {
        BigInteger num = new BigInteger(number);

        if (isPalindrom(num.toString())) {
            num = num.add(BigInteger.ONE);
            if (isPalindrom(num.toString())) {
                return num.toString();
            }
        }
        //                                                          v       v
        int iM = num.toString().length() / 2; // середина числа   12345   1234
        int iPosR = num.toString().length() - 1;

        for (; iPosR >= iM; iPosR--) {
            int iPosL = num.toString().length() - 1 - iPosR;

            char l = num.toString().charAt(iPosL);
            char r = num.toString().charAt(iPosR);

            if (l != r) {
                if (r > l) {
                    // Повышаем разряд числа

                    int zeros = num.toString().length() - iPosR;
                    StringBuilder toAdd = new StringBuilder("1");
                    for (int i = 0; i < zeros; i++) {
                        toAdd.append("0");
                    }
                    num = num.add(new BigInteger(toAdd.toString()));
                }

                // Заменяем цифры с правого конца на цифры [iPosL] с левого, чтобы совпадали
                char[] newString = num.toString().toCharArray();
                newString[iPosR] = num.toString().charAt(iPosL);
                num = new BigInteger(new String(newString));

            }
        }
        return num.toString();
    }

    @Testable
    public static boolean isPalindrom(String num) {
        for (int i = 0; i < num.length() / 2; i++) {
            if (!(num.charAt(i) == num.charAt(num.length() - 1 - i))) {
                return false;
            }
        }
        return true;
    }
}

// All tests passed

    @Test
    void isPalindrom_test() {
        assertTrue(Main.isPalindrom("0"));
        assertTrue(Main.isPalindrom("1"));

        assertTrue(Main.isPalindrom("11"));
        assertTrue(Main.isPalindrom("44"));

        assertTrue(Main.isPalindrom("141"));
        assertTrue(Main.isPalindrom("010"));
        assertTrue(Main.isPalindrom("999"));
        assertTrue(Main.isPalindrom("909"));

        assertTrue(Main.isPalindrom("1234321"));
        assertTrue(Main.isPalindrom("123454321"));
        assertTrue(Main.isPalindrom("99999" + "99999" + "0" + "99999" + "99999"));
        assertTrue(Main.isPalindrom("99999" + "99999" + "00" + "99999" + "99999"));

        assertFalse(Main.isPalindrom("12"));
        assertFalse(Main.isPalindrom("01"));
        assertFalse(Main.isPalindrom("90"));

        assertFalse(Main.isPalindrom("990"));
        assertFalse(Main.isPalindrom("0909"));
        assertFalse(Main.isPalindrom("4949"));

        assertFalse(Main.isPalindrom("99999" + "99999" + "0" + "99999" + "99999" + "1"));
        assertFalse(Main.isPalindrom("1" + "99999" + "99999" + "0" + "99999" + "99999"));
        assertFalse(Main.isPalindrom("1" + "99999" + "99999" + "01" + "99999" + "99999"));
    }

    @Test
    void nextPalindrom_test() {
        assertEquals("2", Main.nextPalindrom("1"));
        assertEquals("1", Main.nextPalindrom("0"));

        assertEquals("11", Main.nextPalindrom("9"));
        assertEquals("11", Main.nextPalindrom("10"));
        assertEquals("22", Main.nextPalindrom("11"));
        assertEquals("22", Main.nextPalindrom("15"));
        assertEquals("22", Main.nextPalindrom("21"));

        assertEquals("33", Main.nextPalindrom("22"));
        assertEquals("33", Main.nextPalindrom("23"));
        assertEquals("33", Main.nextPalindrom("32"));

        assertEquals("101", Main.nextPalindrom("99"));
        assertEquals("101", Main.nextPalindrom("100"));

        assertEquals("11111", Main.nextPalindrom("11101"));
        assertEquals("11111", Main.nextPalindrom("11109"));

        assertEquals("11011", Main.nextPalindrom("11000"));

        assertEquals("10000022000001", Main.nextPalindrom("10000011000001"));
        assertEquals("567909765", Main.nextPalindrom("567898765"));
        assertEquals("567766667765", Main.nextPalindrom("567765567765"));
    }