Número con suma par de dígitos

Un entero positivo se considera un buen número si la suma de sus dígitos es par. Encuentre el n-ésimo número bueno más pequeño. 
Ejemplos: 
 

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

Una solución simple es comenzar desde 1 y atravesar todos los números naturales. Para cada número x, verifica si la suma de los dígitos es par. Si incluso incrementa el conteo de buenos números. Finalmente devuelva el n-ésimo número bueno.
Una solución eficiente se basa en un patrón en la respuesta. Hagamos una lista de los primeros 20 buenos números. Los primeros 20 números buenos son: 2, 4, 6, 8, 11, 13, 15, 17, 19, 20, 22, 24, 26, 28, 31, 33, 35, 37, 39, 40. Observa que si el último dígito de n es de 0 a 4, la respuesta es 2*n y si el último dígito de n es de 5 a 9, la respuesta es 2*n + 1. 
 

// 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;
}

// 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

# 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

// 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.

<?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.
?>

<script>
 
// JavaScript program to find n-th
// Good number.
 
    // Function to find kth good number.
    function findKthGoodNo(n)
    {
           
        // Find the last digit of n.
        let 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
        let n = 10;
        document.write(findKthGoodNo(n));
 
// This code is contributed by souravghosh0416.
</script>

20

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