二十边形数

给定一个数字 n，任务是找到第 n 个二十边形数字。
二十边形数是 20-gon 是一个二十边形的多边形。从比喻类派生的数字。这个号码有不同的图案系列号码。这些点是可数的，以特定的位置方式排列并创建一个图表。所有的点都有一个公共点，所有其他点都连接到这个点，除了这个公共点之外，点连接到它们各自的连续层的第 i 个点。
例子 ：

Input : 3 
Output :57
Input :8 
Output :512 
 

第 n 个二十边形数的公式：

$\begin{math} Ig_{n}=((18n^2)-16n)/2 \end{math}$

C++

// C++ program to find
// nth Icosagonal number
#include 
using namespace std;
 
// Function to calculate Icosagonal number
int icosagonal_poly(long int n)
{
    // Formula for finding
    // nth Icosagonal number
    return (18 * n * n - 16 * n) / 2;
}
 
// Drivers code
int main()
{
    long int n = 7;
    cout << n << "th Icosagonal number :"
               << icosagonal_poly(n);
 
    return 0;
}

Java

// Java program to find
// nth Icosagonal number
 
import java.io.*;
 
class GFG {
     
// Function to calculate Icosagonal number
 
static int icosagonal_poly(int n)
{
    // Formula for finding
    // nth Icosagonal number
    return (18 * n * n - 16 * n) / 2;
}
 
// Drivers code
     
    public static void main (String[] args) {
    
    int n = 7;
     
    System.out.print (n + "th Icosagonal number :");
    System.out.println(icosagonal_poly(n));
    }
}
// This code is contributed by aj_36

Python 3

# Python 3 program to find
# nth Icosagonal number
 
# Function to calculate
# Icosagonal number
def icosagonal_poly(n) :
     
    # Formula for finding
    # nth Icosagonal number
    return (18 * n * n -
            16 * n) // 2
 
# Driver Code
if __name__ == '__main__' :
    n = 7
    print(n,"th Icosagonal number : ",
                   icosagonal_poly(n))
 
# This code is contributed m_kit

C#

// C# program to find
// nth Icosagonal number
using System;
 
class GFG
{
 
// Function to calculate
// Icosagonal number
static int icosagonal_poly(int n)
{
    // Formula for finding
    // nth Icosagonal number
    return (18 * n * n -
            16 * n) / 2;
}
 
// Driver code
static public void Main ()
{
     
int n = 7;
 
Console.Write(n + "th Icosagonal " +
                        "number :");
Console.WriteLine(icosagonal_poly(n));
}
}
 
// This code is contributed by ajit

Javascript

    // Javascript program to find nth Icosagonal number
     
    // Function to calculate
    // Icosagonal number
    function icosagonal_poly(n)
    {
        // Formula for finding
        // nth Icosagonal number
        return (18 * n * n - 16 * n) / 2;
    }
     
    let n = 7;
   
    document.write(n + "th Icosagonal number :");
    document.write(icosagonal_poly(n));
     

输出 ：

7th Icosagonal number :385

时间复杂度： O(1)
辅助空间： O(1)

参考：https://en.wikipedia.org/wiki/Polygonal_number

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