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Compute n! under modulo p

Last Updated : 23 Jul, 2025

Given a large number n and a prime p, how to efficiently compute n! % p?
Examples : 
 

Input: n = 5, p = 13
Output: 3
5! = 120 and 120 % 13 = 3

Input: n = 6, p = 11
Output: 5
6! = 720 and 720 % 11 = 5


A Naive Solution is to first compute n!, then compute n! % p. This solution works fine when the value of n! is small. The value of n! % p is generally needed for large values of n when n! cannot fit in a variable, and causes overflow. So computing n! and then using modular operator is not a good idea as there will be overflow even for slightly larger values of n and r. 
Following are different methods.
Method 1 (Simple) 
A Simple Solution is to one by one multiply result with i under modulo p. So the value of result doesn't go beyond p before next iteration.
 

Output : 

5


Time Complexity : O(n).

Auxiliary Space : O(1) since using only constant space


Method 2 (Using Sieve) 
The idea is based on below formula discussed here

The largest possible power of a prime pi that divides n is,
 ?n/pi? + ?n/(pi2)? + ?n/(pi3)? + .....+ 0 

Every integer can be written as multiplication of powers of primes. So,
 n! = p1k1 * p2k2 * p3k3 * ....
 Where p1, p2, p3, .. are primes and 
 k1, k2, k3, .. are integers greater than or equal to 1.


The idea is to find all primes smaller than n using Sieve of Eratosthenes. For every prime 'pi', find the largest power of it that divides n!. Let the largest power be ki. Compute piki % p using modular exponentiation. Multiply this with final result under modulo p.
Below is implementation of above idea.
 

Output: 

5

Time Complexity: O(nlog(logn)) 

Auxiliary Space: O(n)


This is an interesting method, but time complexity of this is more than Simple Method as time complexity of Sieve itself is O(n log log n). This method can be useful if we have list of prime numbers smaller than or equal to n available to us.


Method 3 (Using Wilson's Theorem) 
Wilson’s theorem states that a natural number p > 1 is a prime number if and only if 

 (p - 1) ! ? -1 mod p 
OR (p - 1) ! ? (p-1) mod p 


Note that n! % p is 0 if n >= p. This method is mainly useful when p is close to input number n. For example (25! % 29). From Wilson's theorem, we know that 28! is -1. So we basically need to find [ (-1) * inverse(28, 29) * inverse(27, 29) * inverse(26) ] % 29. The inverse function inverse(x, p) returns inverse of x under modulo p (See this for details). 
 

Output: 

5


Time complexity of this method is O((p-n)*Logn)

Auxiliary Space : O(1) because using only constant variables


Method 4 (Using Primality Test Algorithm) 

1) Initialize: result = 1
2) While n is not prime
 result = (result * n) % p
3) result = (result * (n-1)) % p // Using Wilson's Theorem 
4) Return result.


Note that time complexity step 2 of above algorithm depends on the primality test algorithm being used and value of the largest prime smaller than n. The AKS algorithm for example takes O(Log 10.5 n) time. 
 

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