十进制数

给定一个数 n，任务是找到第 n 个十进制数。
十九边形数在数学中是一个 19 边形的多边形。它属于一类具象数字。数字包含点的数量，并且点以图案或系列排列。十九边形数也称为十九边形。这些点具有公共点，所有其他点都排列在连续层中。

例子 ：

Input : 4 
Output :106
Input :10 
Output :775 

求第 n 个十进制数的公式：

$\begin{math} Ed_{n}=((17n^2)-15n)/2 \end{math}$

C++

// C++ program to find
// nth Enneadecagonal number
#include 
using namespace std;
 
// Function to calculate
// Enneadecagonal number
int nthEnneadecagonal(long int n)
{
    // Formula for finding
    // nth Enneadecagonal number
    return (17 * n * n - 15 * n) / 2;
}
 
// Drivers code
int main()
{
    long int n = 6;
    cout << n << "th Enneadecagonal number :" << nthEnneadecagonal(n);
    return 0;
}

Java

// Java program to find
// nth Enneadecagonal number
import java.io.*;
 
class GFG {
 
    // Function to calculate
    // Enneadecagonal number
    static int nthEnneadecagonal(int n)
    {
         
        // Formula for finding
        // nth Enneadecagonal number
        return (17 * n * n - 15 * n) / 2;
    }
     
    // Driver Code
    public static void main (String[] args)
    {
         
        int n = 6;
        System.out.print(n + "th Enneadecagonal number :");
     
        System.out.println( nthEnneadecagonal(n));
    }
}
 
// This code is contributed by m_kit.

Python3

# Program to find nth
# Enneadecagonal number
 
def nthEnneadecagonal(n) :
     
    # Formula to calculate nth
    # Enneadecagonal number
    return (17 * n * n - 15 * n) // 2
 
# Driver Code
if __name__ == '__main__' :
         
    n = 6
    print(n,"th Enneadecagonal number :"
                , nthEnneadecagonal(n))
 
# This code is contributed  by Ajit

C#

// C# program to find
// nth Enneadecagonal number
using System;
 
class GFG
{
    // Function to calculate
    // Enneadecagonal number
    static int nthEnneadecagonal(int n)
    {
         
    // Formula for finding
    // nth Enneadecagonal number
    return (17 * n * n - 15 * n) / 2;
    }
     
    // Driver Code
    static public void Main ()
    {
    int n = 6;
    Console.Write(n + "th Enneadecagonal number :");
     
    Console.WriteLine( nthEnneadecagonal(n));
    }
}
 
// This code is contributed by aj_36

Javascript

    // Javascript program to find nth Enneadecagonal number
     
    // Function to calculate
    // Enneadecagonal number
    function nthEnneadecagonal(n)
    {
           
        // Formula for finding
        // nth Enneadecagonal number
        return (17 * n * n - 15 * n) / 2;
    }
     
    let n = 6;
    document.write(n + "th Enneadecagonal number :");
 
    document.write( nthEnneadecagonal(n));
     

输出：

6th Enneadecagonal number :261

时间复杂度： O(1)
辅助空间： O(1)

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