找到系列 0、2、6、12、20、30、42... 的第 N 项

给定一个正整数N ，任务是找到序列的第 N 项

0, 2, 6, 12, 20…till N terms

例子：

Input: N = 7
Output: 42

Input: N = 10
Output: 90

方法：

从给定的系列中，找到第 N项的公式-

1st term = 1 * (1 – 1) = 0

2nd term = 2 * (2 – 1) = 2

3rd term = 3 * (3 – 1) = 6

4th term = 4 * (4 – 1) = 12

.

.

Nth term = N * (N – 1)

给定系列的第 N项可以概括为-

T_N = N * (N – 1)

插图：

Input: N = 7
Output: 42
Explanation:
T_N = N * (N – 1)
     = 7 * (7 – 1)
     = 7 * 6
     = 42

以下是上述方法的实现 -

C++

// C++ program to implement
// the above approach
#include 
using namespace std;
 
// Function to return
// Nth term of the series
int nthTerm(int n)
{
    return n * n - n;
}
 
// Driver code
int main()
{
    // Value of N
    int N = 7;
    cout << nthTerm(N) << endl;
    return 0;
}

C

// C program to implement
// the above approach
#include 
 
// Function to return
// Nth term of the series
int nthTerm(int n)
{
    return n * n - n;
}
 
// Driver code
int main()
{
    // Value of N
    int N = 7;
    printf("%d", nthTerm(N));
    return 0;
}

Java

// Java program to implement
// the above approach
import java.io.*;
 
class GFG {
    public static void main(String[] args)
    {
        // Value of N
        int N = 7;
        System.out.println(nthTerm(N));
    }
 
    // Function to return
    // Nth term of the series
    public static int nthTerm(int n)
    {
        return n * n - n;
    }
}

Python3

# Python code for the above approach
 
# Function to return
# Nth term of the series
def nthTerm(n):
    return n * n - n;
 
# Driver code
 
# Value of N
N = 7;
print(nthTerm(N));
 
# This code is contributed by Saurabh Jaiswal

C#

using System;
 
public class GFG
{
 
  // Function to return
  // Nth term of the series
  public static int nthTerm(int n)
  {
    return n * n - n;
  }
   
  static public void Main()
  {
 
    // Code
    // Value of N
    int N = 7;
    Console.Write(nthTerm(N));
  }
}
 
// This code is contributed by Potta Lokesh

Javascript

        // JavaScript code for the above approach
 
 
        // Function to return
        // Nth term of the series
        function nthTerm(n) {
            return n * n - n;
        }
 
        // Driver code
 
        // Value of N
        let N = 7;
        document.write(nthTerm(N) + '<br>');
 
       // This code is contributed by Potta Lokesh
    

输出

42

时间复杂度： O(1)
辅助空间： O(1)