中心八边形数

给定一个数 n，找出第n 个中心八边形数。
Centered Octadecagonal Number表示中心的一个点，其他点围绕它排列在连续的十八边形（18 边多边形）层中。

例子 ：

Input :  2
Output : 19

Input : 6
Output : 271

在数学中，第 n项的中心八边形数由下式给出：

$CO_{n}= 9n^2 -9n+1$

下面是上述想法的基本实现：

C++

// C++ Program to find the
// nth centered octadecagonal
// number
#include 
using namespace std;
 
// centered octadecagon  function
int center_octadecagon_num(long int n)
{
    // Formula to calculate nth
    // centered octadecagonal number
    return 9 * n * n - 9 * n + 1;
}
 
// Driver Code
int main()
{
    long int n = 3;
    cout << n << "th centered octadecagonal number : "
                  << center_octadecagon_num(n);
    cout << endl;
    n = 13;
    cout << n << "th centered octadecagonal number : "
                 << center_octadecagon_num(n);
 
    return 0;
}

Java

// Java Program to find the
// nth centered octadecagonal
// number
import java.io.*;
 
class GFG
{
     
// centered octadecagon function
static int center_octadecagon_num(int n)
{
         
    // Formula to calculate nth
    // centered octadecagonal number
    return 9 * n * n - 9 * n + 1;
}
 
// Driver Code
public static void main (String[] args)
{
int n = 3;
System.out.print(n + "th centered " +
           "octadecagonal number : ");
System.out.println(center_octadecagon_num(n));
 
n = 13;
System.out.print(n + "th centered " +
           "octadecagonal number : ");
System.out.println(center_octadecagon_num(n));
}
}
 
// This code is contributed by ajit

Python3

# Program to find nth
# centered octadecagonal number
 
# Centered octadecagonal
# number function
def center_octadecagon_num(n) :
     
    # Formula to calculate
    # nth centered octadecagonal
    # number & return it
    # into main function.
    return(9 * n * n -
           9 * n + 1)
 
# Driver Code
if __name__ == '__main__' :
         
    n = 3
    print(n,"rd centered octadecagonal " +
                                "number : ",
                center_octadecagon_num(n))
 
    n = 13
    print(n,"th centered octadecagonal " +
                              "number : ",
                center_octadecagon_num(n))
             
# This code is contributed
# by akt_mit

C#

// C# Program to find the
// nth centered octadecagonal
// number
using System;
 
class GFG
{
     
// centered octadecagon function
static int center_octadecagon_num(int n)
{
         
    // Formula to calculate nth
    // centered octadecagonal number
    return 9 * n * n - 9 * n + 1;
}
 
// Driver Code
static public void Main ()
{
     
    int n = 3;
    Console.Write( n + "th centered " +
            "octadecagonal number : ");
    Console.WriteLine( center_octadecagon_num(n));
 
    n = 13;
    Console.Write( n + "th centered " +
            "octadecagonal number : ");
    Console.WriteLine(center_octadecagon_num(n));
}
}
 
// This code is contributed by aj_36.

Javascript

 
// Javascript program to find the
// nth centered octadecagonal
// number
 
// centered octadecagon function
function center_octadecagon_num(n)
{
     
    // Formula to calculate nth
    // centered octadecagonal number
    return 9 * n * n - 9 * n + 1;
}
 
 
// Driver Code
var n = 3;
document.write(n + "th centered " +
           "octadecagonal number : ");
document.write(center_octadecagon_num(n) + "<br>");
 
n = 13;
document.write(n + "th centered " +
           "octadecagonal number : ");
document.write(center_octadecagon_num(n));
 
// This code is contributed by Kirti
     

输出 ：

3th centered octadecagonal number : 55
13th centered octadecagonal number : 1405

时间复杂度： O(1)
辅助空间： O(1)

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