# What is the time complexity of nCr?

## What is the time complexity of nCr?

A naive approach is to calculate nCr using formulae by applying modular operations at any time. Hence time complexity will be O(q*n).

How do you calculate nCr mod?

We know the formula for nCr nCr = fact(n) / (fact(r) x fact(n-r)) Here fact() means factorial. Cr % p = (fac[n]* modIverse(fac[r]) % p * modIverse(fac[n-r]) % p) % p; Here modIverse() means modular inverse under modulo p. Following is the implementation of the above algorithm.

### How do you calculate nCr fast?

The fastest method I know is Vladimir’s method. One avoids division all together by decomposing nCr into prime factors. As Vladimir says you can do this pretty efficiently using Eratosthenes sieve. Also,Use Fermat’s little theorem to calculate nCr mod MOD(Where MOD is a prime number).

How do you do nCr problems?

How Do you Use NCR Formula in Probability? Combinations are a way to calculate the total number of outcomes of an event when the order of the outcomes does not matter. To calculate combinations we use the nCr formula: nCr = n! / r! * (n – r)!, where n = number of items, and r = number of items being chosen at a time.

#### Can nCr be decimal?

Yes. For example: 2=6/3. 5=10/2.

What is inverse of a mod m?

The modular multiplicative inverse is an integer ‘x’ such that. The multiplicative inverse of “a modulo m” exists if and only if a and m are relatively prime (i.e., if gcd(a, m) = 1). Examples: Input: a = 3, m = 11 Output: 4 Since (4*3) mod 11 = 1, 4 is modulo inverse of 3(under 11).

## For which primes p does m not have an inverse mod p?

Thus if m is prime, the only number between 0 and m-1 that does not have a mod m multiplicative inverse is 0.

Can N be negative NCR?

It is valid to have a negative value for “n” and the first value you calculated is correct but the second value is not correct. Note: The notation is equivalent to and they are referred to as binomial coefficients. 