Find subfactorial of a number

Given an integer N, the task is to find the subfactorial of the number represented as !N. The subfactorial of a number is defined using below recurrence relation of a number N:

!N = (N-1) [ !(N-2) + !(N-1) ]

where !1 = 0 and !0 = 1

Some of the subfactorials are:

Examples:

Input: N = 4
Output: 9
Explanation: 
!4 = !(4-1)*4 + (-1)⁴ = !3*4 + 1
!3 = !(3 - 1)*3 + (-1)³ = !2*3 - 1
!2 = !(2 - 1)*2 + (-1)² = !1*2 + 1
!1 = !(1 - 1)*1 + (-1)¹ = !0*1 - 1
Since !0 = 1, therefore !1 = 0, !2 = 1, !3 = 2 and !4 = 9. 

Input: N = 0
Output: 1

Approach: The subfactorial of the number N can also be calculated as:

{\displaystyle !n={\begin{cases}1&{\text{if }}n=0, \\n\left(!(n-1)\right)+(-1)^{n}&{\text{if }}n>0.\end{cases}}}

Expanding this gives

{\displaystyle !n=\sum _{i=0}^{n-1}i(!i)+{\frac {1+(-1)^{n}}{2}}}

=> !N = ( N! )*( 1 - 1/(1!) + (1/2!) - (1/3!)  ........ (1/N!)*(-1)^N )

Therefore the above series can be used to find the subfactorial of number N. Follow the steps below to see how:

• Initialize variables, say res = 0, fact = 1 and count = 0.
• Iterate over the range from 1 to N using i and do the following:
  • Update fact as fact*i.
  • If the count is even then update res as res = res - (1 / fact).
  • If the count is odd then update res as res = res + (1 / fact).
  • Increase the value of count by 1.
• Finally, return fact*(1 + res).

Below is the implementation of the above approach:

/// C++ program for the above approach
#include <iostream>
using namespace std;

// Function to find the subfactorial
// of the number
double subfactorial(int N)
{

    // Initialize variables
    double res = 0, fact = 1;
    int count = 0;

    // Iterating over range N
    for (int i = 1; i <= N; i++) {

        // Fact variable store
        // factorial of the i
        fact = fact * i;

        // If count is even
        if (count % 2 == 0)
            res = res - (1 / fact);
        else
            res = res + (1 / fact);

        // Increase the value of
        // count by 1
        count++;
    }

    return fact * (1 + res);
}

// Driver Code
int main()
{
    int N = 4;
    cout << subfactorial(N);

    return 0;
}

/// Java program for the above approach
import java.util.*;

class GFG {

    // Function to find the subfactorial
    // of the number
    static double subfactorial(int N)
    {

        // Initialize variables
        double res = 0, fact = 1;
        int count = 0;

        // Iterating over range N
        for (int i = 1; i <= N; i++) {

            // Fact variable store
            // factorial of the i
            fact = fact * i;

            // If count is even
            if (count % 2 == 0)
                res = res - (1 / fact);
            else
                res = res + (1 / fact);

            // Increase the value of
            // count by 1
            count++;
        }

        return fact * (1 + res);
    }

    // Driver Code
    public static void main(String[] args)
    {
        int N = 4;
        System.out.println((int)(subfactorial(N)));
    }
}

// This code is contributed by ukasp.

# python program for the above approach

# Function to find the subfactorial
# of the number
def subfactorial(N):

        # Initialize variables
    res = 0
    fact = 1
    count = 0

    # Iterating over range N
    for i in range(1, N+1):

                # Fact variable store
                # factorial of the i
        fact = fact * i

        # If count is even
        if (count % 2 == 0):
            res = res - (1 / fact)

        else:
            res = res + (1 / fact)

            # Increase the value of
            # count by 1
        count += 1

    return fact * (1 + res)

# Driver Code
if __name__ == "__main__":

    N = 4
    print(subfactorial(N))

    # This code is contributed by rakeshsahni

/// C# program for the above approach
using System;
using System.Collections.Generic;

class GFG{

// Function to find the subfactorial
// of the number
static double subfactorial(int N)
{

    // Initialize variables
    double res = 0, fact = 1;
    int count = 0;

    // Iterating over range N
    for (int i = 1; i <= N; i++) {

        // Fact variable store
        // factorial of the i
        fact = fact * i;

        // If count is even
        if (count % 2 == 0)
            res = res - (1 / fact);
        else
            res = res + (1 / fact);

        // Increase the value of
        // count by 1
        count++;
    }

    return fact * (1 + res);
}

// Driver Code
public static void Main()
{
    int N = 4;
    Console.Write(subfactorial(N));
}
}

// This code is contributed by ipg2016107.

<script>
        // JavaScript Program to implement
        // the above approach

        // Function to find the subfactorial
        // of the number
        function subfactorial(N) {

            // Initialize variables
            let res = 0, fact = 1;
            let count = 0;

            // Iterating over range N
            for (let i = 1; i <= N; i++) {

                // Fact variable store
                // factorial of the i
                fact = fact * i;

                // If count is even
                if (count % 2 == 0)
                    res = res - 1 / fact;
                else
                    res = res + 1 / fact;

                // Increase the value of
                // count by 1
                count++;
            }

            return fact * (1 + res);
        }

        // Driver Code
        let N = 4;
        document.write(subfactorial(N));

// This code is contributed by Potta Lokesh
    </script>

9

Time Complexity: O(N)
Auxiliary Space: O(1)