求系列 3, 7, 14, 27, 52, 的第 N 项。 . .

给定一个正整数N 。任务是找到系列3, 7, 14, 27, 52, …..的第 N项

例子：

Input: N = 5
Output: 52

Input: N = 1
Output: 3

方法：

该序列是通过使用以下模式形成的。对于任何值 N-

T_N = (N -1) + 3 * 2^N-1

插图：

Input: N = 5
Output: 52
Explanation: 
T_N = (5 – 1) + 3 * 2^{5 – 1}
       = 4 + 3 * 16
     = 52
 

下面是上述方法的实现：

C++

// C++ program to implement
// the above approach
#include 
using namespace std;
 
// Function to return Nth term
// of the series
int calcNum(int N)
{
    return ((N - 1) + 3 *
             pow(2, N - 1));
}
 
// Driver Code
int main()
{
    int N = 5;
    cout << calcNum(N);
    return 0;
}

Java

// Java program to implement
// the above approach
class GFG
{
   
    // Function to return Nth term
    // of the series
    static int calcNum(int N) {
        return (int) ((N - 1) + 3 * Math.pow(2, N - 1));
    }
 
    // Driver Code
    public static void main(String args[]) {
        int N = 5;
        System.out.println(calcNum(N));
    }
}
 
// This code is contributed by saurabh_jaiswal.

Python3

# Python code for the above approach
 
# Function to return Nth term
# of the series
def calcNum(N):
    return ((N - 1) + 3 * (2 ** (N - 1)));
 
# Driver Code
N = 5;
print(calcNum(N));
 
# This code is contributed by Saurabh Jaiswal

C#

// C# program to implement
// the above approach
using System;
class GFG {
 
    // Function to return Nth term
    // of the series
    static int calcNum(int N)
    {
        return (int)((N - 1) + 3 * Math.Pow(2, N - 1));
    }
 
    // Driver Code
    public static void Main()
    {
        int N = 5;
        Console.WriteLine(calcNum(N));
    }
}
 
// This code is contributed by ukasp.

Javascript

      // JavaScript code for the above approach
 
      // Function to return Nth term
      // of the series
      function calcNum(N) {
          return ((N - 1) + 3 *
              Math.pow(2, N - 1));
      }
 
      // Driver Code
      let N = 5;
      document.write(calcNum(N));
 
// This code is contributed by Potta Lokesh
  

输出

52

时间复杂度： O(1)

辅助空间： O(1)