Generate all palindromic numbers less than n

24 Jan, 2022
Find all numbers less than n, which are palindromic. Numbers can be printed in any order.
Examples : 
Input : n = 12
Output : 1, 2, 3, 4, 5, 6, 7, 8, 9, 11

Input : n = 104
Output : 1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 
         22, 33, 44, 55, 66, 77, 88, 99, 101
[Note that below program prints these numbers
 in different order]
 
Brute Force: We check all the numbers from 1 to n whether its decimal representation is palindrome or not. 
Efficient Approach: We start from 1 and create palindromes of odd digit and even digits up to n. For every number (starting from 1), we append its reverse at end if we need even length palindrome numbers. For odd length palindrome, we append reverse of all digits except last one. 
C++
Java
Python3
C#
PHP
Javascript
// A C++ program to generate palindromic numbers// less than n.#include <iostream>using namespace std; // A utility for creating palindromeint createPalindrome(int input, int b, bool isOdd){    int n = input;    int palin = input;     // checks if number of digits is odd or even    // if odd then neglect the last digit of input in    // finding reverse as in case of odd number of    // digits middle element occur once    if (isOdd)        n /= b;     // Creates palindrome by just appending reverse    // of number to itself    while (n > 0)    {        palin = palin * b + (n % b);        n /= b;    }    return palin;} // Function to print decimal palindromic numbervoid generatePalindromes(int n){    int number;     // Run two times for odd and even length palindromes    for (int j = 0; j < 2; j++)    {        // Creates palindrome numbers with first half as i.        // Value of j decided whether we need an odd length        // of even length palindrome.        int i = 1;        while ((number = createPalindrome(i, 10, j % 2)) < n)        {            cout << number << " ";            i++;        }    }} // Driver Program to test above functionint main(){    int n = 104;    generatePalindromes(n);    return 0;}
// A Java program to generate palindromic// numbers less than n.class GFG { // A utility for creating palindromestatic int createPalindrome(int input, int b, int isOdd) {    int n = input;    int palin = input;     // checks if number of digits is odd or even    // if odd then neglect the last digit of input in    // finding reverse as in case of odd number of    // digits middle element occur once    if (isOdd == 1)        n /= b;     // Creates palindrome by just appending reverse    // of number to itself    while (n > 0) {        palin = palin * b + (n % b);        n /= b;    }    return palin;} // Function to print decimal// palindromic numberstatic void generatePalindromes(int n) {    int number;     // Run two times for odd and even    // length palindromes    for (int j = 0; j < 2; j++) {     // Creates palindrome numbers with first    // half as i. Value of j decided whether    // we need an odd length of even length    // palindrome.        int i = 1;        while ((number = createPalindrome(i, 10, j % 2)) < n) {            System.out.print(number + " ");            i++;    }    }} // Driver codepublic static void main(String[] args) {    int n = 104;    generatePalindromes(n);}}// This code is contributed by Anant Agarwal.
# Generate all palindromic numbers less than n# A Python program to generate palindromic numbers# less than n.def createPalindrome(inp, b, isOdd):    n = inp    palin = inp      # checks if number of digits is odd or even    # if odd then neglect the last digit of input in    # finding reverse as in case of odd number of    # digits middle element occur once    if (isOdd):        n = n // b      # Creates palindrome by just appending reverse    # of number to itself    while (n > 0):        palin = palin * b + (n % b)        n = n // b    return palin  # Function to print decimal palindromic numberdef generatePalindromes(n):      # Run two times for odd and even length palindromes    for j in range(2):        # Creates palindrome numbers with first half as i.        # Value of j decided whether we need an odd length        # of even length palindrome.        i = 1        while (createPalindrome(i, 10, j % 2) < n):            print (createPalindrome(i, 10, j % 2),end=" ")            i = i + 1  # Driver Program to test above functionn = 104generatePalindromes(n) #This code is contributed by Afzal Ansari
// A C# program to generate palindromic// numbers less than n.using System; class GFG { // A utility for creating palindromestatic int createPalindrome(int input, int b,                            int isOdd){    int n = input;    int palin = input;     // checks if number of digits is odd    // or even if odd then neglect the    // last digit of input in finding reverse    // as in case of odd number of digits    // middle element occur once    if (isOdd == 1)        n /= b;     // Creates palindrome by just appending    // reverse of number to itself    while (n > 0)    {        palin = palin * b + (n % b);        n /= b;    }    return palin;} // Function to print decimal// palindromic numberstatic void generatePalindromes(int n){    int number;     // Run two times for odd and even    // length palindromes    for (int j = 0; j < 2; j++)    {         // Creates palindrome numbers with first        // half as i. Value of j decided whether        // we need an odd length of even length        // palindrome.        int i = 1;        while ((number = createPalindrome(i, 10,                                    j % 2)) < n)        {            Console.Write(number + " ");            i++;    }    }} // Driver Codepublic static void Main(){    int n = 104;    generatePalindromes(n);}} // This code is contributed by Nitin Mittal.
<?php// PHP program to generate// palindromic numbers less than n. // A utility function for// creating palindromefunction createPalindrome($input,                          $b, $isOdd){    $n = $input;    $palin = $input;     // checks if number of digits is    // odd or even if odd then neglect    // the last digit of input in finding    // reverse as in case of odd number    // of digits middle element occur once    if ($isOdd)        $n = intval($n / $b);     // Creates palindrome by just appending    // reverse of number to itself    while ($n > 0)    {        $palin = $palin * $b + intval($n % $b);        $n = intval($n / $b);    }    return $palin;} // Function to print decimal// palindromic numberfunction generatePalindromes($n){    $number = 0;     // Run two times for odd and    // even length palindromes    for ($j = 0; $j < 2; $j++)    {        // Creates palindrome numbers        // with first half as i. Value        // of j decided whether we need        // an odd length of even length        // palindrome.        $i = 1;        while (($number =                   createPalindrome($i, 10,                                    $j % 2)) < $n)        {            echo $number . " ";            $i++;        }    }} // Driver Code$n = 104;generatePalindromes($n);     // This code is contributed by Sam007?>
<script>    // A Javascript program to generate palindromic    // numbers less than n.         // A utility for creating palindrome    function createPalindrome(input, b, isOdd)    {        let n = input;        let palin = input;         // checks if number of digits is odd        // or even if odd then neglect the        // last digit of input in finding reverse        // as in case of odd number of digits        // middle element occur once        if (isOdd == 1)            n = parseInt(n / b, 10);         // Creates palindrome by just appending        // reverse of number to itself        while (n > 0)        {            palin = palin * b + (n % b);            n = parseInt(n / b, 10);        }        return palin;    }     // Function to print decimal    // palindromic number    function generatePalindromes(n)    {        let number;         // Run two times for odd and even        // length palindromes        for (let j = 0; j < 2; j++)        {             // Creates palindrome numbers with first            // half as i. Value of j decided whether            // we need an odd length of even length            // palindrome.            let i = 1;            while ((number = createPalindrome(i, 10, j % 2)) < n)            {                document.write(number + " ");                i++;            }        }    }         let n = 104;    generatePalindromes(n);       // This code is contributed by divyesh072019.</script>
Output : 
11 22 33 44 55 66 77 88 99 1 2 3 4 5 6 7 8 9 101 
Note that the above program doesn’t print output in sorted order. To print in sorted order, we can store palindromes in a vector and sort it, and don’t forget to use the required header file.
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