Number with even sum of digits

A positive integer is considered a good number if sum of its digits is even. Find n-th smallest good number.

Examples :

Input :  n = 1
Output : 2
First good number is smallest positive
number with sum of digits even which is 2.

Input : n = 10
Output : 20

A simple solution is to start from 1 and traverse through all natural numbers. For every number x, check if sum of digits is even. If even increment count of good numbers. Finally return the n-th Good number.



An efficient solution is based on a pattern in the answer. Let us list down first 20 good numbers. The first 20 good numbers are: 2, 4, 6, 8, 11, 13, 15, 17, 19, 20, 22, 24, 26, 28, 31, 33, 35, 37, 39, 40. Observe that if last digit of n is from 0 to 4 the answer is 2*n and if last digit of n is from 5 to 9 the answer is 2*n + 1.

C++

filter_none

edit
close

play_arrow

link
brightness_4
code

// C++ program to find n-th
// Good number.
#include <bits/stdc++.h>
using namespace std;
  
// Function to find kth good number.
long long int findKthGoodNo(long long int n)
{
    // Find the last digit of n.
    int lastDig = n % 10;
  
    // If last digit is between 
    // 0 to 4 then return 2 * n.
    if (lastDig >= 0 && lastDig <= 4)
        return n << 1;
  
    // If last digit is between 
    // 5 to 9 then return 2*n + 1.
    else
        return (n << 1) + 1;
}
  
// Driver code
int main()
{
    long long int n = 10;
    cout << findKthGoodNo(n);
    return 0;
}

chevron_right






                        filter_none









Java














                                    filter_none




                                    edit

                                    close




                                    play_arrow




                                    link

                                    brightness_4

                                    code
                                




















// Java program to find n-th 


// Good number. 


class GFG  


{ 


      


    // Function to find kth good number. 


    static int findKthGoodNo(int n) 


    { 


          


        // Find the last digit of n. 


        int lastDig = n % 10; 


  


        // If last digit is between  


        // 0 to 4 then return 2*n. 


        if (lastDig >= 0 && lastDig <= 4) 


            return n << 1; 


  


        // If last digit is between  


        // 5 to 9 then return 2*n + 1. 


        else


            return (n << 1) + 1; 


    } 


  


    // Driver code 


    public static void main(String[] args) 


    { 


        int n = 10; 


          


        System.out.println(findKthGoodNo(n)); 


    } 


} 


  


// This code is contributed by 


// Smitha Dinesh Semwal 



















                        chevron_right







                        filter_none









Python 3














                                    filter_none




                                    edit

                                    close




                                    play_arrow




                                    link

                                    brightness_4

                                    code
                                




















# Python 3 program to find 


# n-th Good number. 


  


# Function to find kth  


# good number. 


def findKthGoodNo(n): 


  


    # Find the last digit of n. 


    lastDig = n % 10


  


    # If last digit is between  


    # 0 to 4 then return 2 * n. 


    if (lastDig >= 0 and lastDig <= 4) : 


        return n << 1


  


    # If last digit is between  


    # 5 to 9 then return 2 * n + 1. 


    else: 


        return (n << 1) + 1


  


  


# Driver code 


n = 10


print(findKthGoodNo(n)) 


  


# This code is contributed by 


# Smitha Dinesh Semwal 



















                        chevron_right







                        filter_none









C#














                                    filter_none




                                    edit

                                    close




                                    play_arrow




                                    link

                                    brightness_4

                                    code
                                




















// C# program to find n-th 


// Good number. 


using System; 


  


class GFG 


{ 


      


    // Function to find kth 


    // good number 


    public static int findKthGoodNo(int n) 


    { 


          


        // Find the last digit of n. 


        int lastDig = n % 10; 


  


        // If last digit is between 


        // 0 to 4 then return 2*n. 


        if (lastDig >= 0 && lastDig <= 4) 


            return n << 1; 


  


        // If last digit is between  


        // 5 to 9 then return 2*n + 1. 


        else


            return (n << 1) + 1; 


    } 


      


    // Driver code 


    static public void Main (string []args) 


    { 


        int n = 10; 


        Console.WriteLine(findKthGoodNo(n)); 


    } 


} 


  


// This code is contributed by Ajit. 



















                        chevron_right







                        filter_none









PHP














                                    filter_none




                                    edit

                                    close




                                    play_arrow




                                    link

                                    brightness_4

                                    code
                                




















<?php 


// PHP program to find n-th  


// Good number. 


  


// Function to find kth  


// good number. 


function findKthGoodNo($n) 


{ 


    // Find the last digit of n. 


    $lastDig = $n % 10; 


  


    // If last digit is between  


    // 0 to 4 then return 2*n. 


    if ($lastDig >= 0 && $lastDig <= 4) 


        return $n << 1; 


  


    // If last digit is between  


    // 5 to 9 then return 2*n + 1. 


    else


        return ($n << 1) + 1; 


} 


  


// Driver code 


$n = 10; 


echo(findKthGoodNo($n)); 


  


// This code is contributed by Ajit. 


?> 



















                        chevron_right







                        filter_none











Output:


20





Time Complexity:  O(1)

Auxiliary Space:  O(1)



          
          
          
          

            

    

        My Personal Notes
        arrow_drop_up
    

    

        
        

            
            
        

    


Recommended Posts:
nik1996
A Programmer and A Machine learning Enthusiast
If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please Improve this article if you find anything incorrect by clicking on  the "Improve Article" button below.