# Ways to write N as sum of two or more positive integers | Set-2

• Last Updated : 12 Apr, 2021

Given a number N, the task is to find the number of ways N can be partitioned, i.e. the number of ways that N can be expressed as a sum of positive integers.
Note: N should also be considered itself a way to express it as a sum of positive integers.
Examples:

Input: N = 5
Output:
5 can be partitioned in the following ways:

4 + 1
3 + 2
3 + 1 + 1
2 + 2 + 1
2 + 1 + 1 + 1
1 + 1 + 1 + 1 + 1
Input: N = 10
Output: 42

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This post has been already discussed in Ways to write n as sum of two or more positive integers. In this post, an efficient approach is discussed.
Approach(Using Euler’s recurrence):
If p(n) is the number of partitions of N, then it can be generated by the following generating function: Using this formula and Euler’s pentagonal number theorem, we can derive the following recurrence relation for p(n): (Check the Wikipedia article for more details) where k = 1, -1, 2, -2, 3, -3, … and p(n) = 0 for n < 0.
Below is the implementation of above approach:

## C++

 // C++ implementation of above approach#include using namespace std; // Function to find the number// of partitions of Nlong long partitions(int n){    vector<long long> p(n + 1, 0);     // Base case    p = 1;     for (int i = 1; i <= n; ++i) {        int k = 1;        while ((k * (3 * k - 1)) / 2 <= i) {            p[i] += (k % 2 ? 1 : -1) * p[i - (k * (3 * k - 1)) / 2];             if (k > 0)                k *= -1;            else                k = 1 - k;        }    }     return p[n];} // Driver codeint main(){    int N = 20;    cout << partitions(N);    return 0;}

## Java

 // Java implementation of above approachclass GFG{     // Function to find the number    // of partitions of N    static long partitions(int n)    {        long p[] = new long[n + 1];         // Base case        p = 1;         for (int i = 1; i <= n; ++i)        {            int k = 1;            while ((k * (3 * k - 1)) / 2 <= i)            {                p[i] += (k % 2 != 0 ? 1 : -1) *                    p[i - (k * (3 * k - 1)) / 2];                 if (k > 0)                {                    k *= -1;                }                else                {                    k = 1 - k;                }            }        }        return p[n];    }     // Driver code    public static void main(String[] args)    {        int N = 20;        System.out.println(partitions(N));    }} // This code is contributed by Rajput-JI

## Python 3

 # Python 3 implementation of# above approach # Function to find the number# of partitions of Ndef partitions(n):     p =  * (n + 1)     # Base case    p = 1     for i in range(1, n + 1):        k = 1        while ((k * (3 * k - 1)) / 2 <= i) :            p[i] += ((1 if k % 2 else -1) *                    p[i - (k * (3 * k - 1)) // 2])             if (k > 0):                k *= -1            else:                k = 1 - k     return p[n] # Driver codeif __name__ == "__main__":    N = 20    print(partitions(N)) # This code is contributed# by ChitraNayal

## C#

 // C# implementation of above approachusing System; class GFG{     // Function to find the number    // of partitions of N    static long partitions(int n)    {        long []p = new long[n + 1];         // Base case        p = 1;         for (int i = 1; i <= n; ++i)        {            int k = 1;            while ((k * (3 * k - 1)) / 2 <= i)            {                p[i] += (k % 2 != 0 ? 1 : -1) *                    p[i - (k * (3 * k - 1)) / 2];                 if (k > 0)                {                    k *= -1;                }                else                {                    k = 1 - k;                }            }        }        return p[n];    }     // Driver code    public static void Main(String[] args)    {        int N = 20;        Console.WriteLine(partitions(N));    }} // This code has been contributed by 29AjayKumar

## PHP

  0)                $k *= -1; else $k = 1 - $k; } } return $p[$n];} // Driver Code$N = 20;print(partitions(\$N)); // This code is contributed// by mits?>

## Javascript

 
Output:
627

Time Complexity: O(N√N)
Space Complexity: O(N)

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