Write a function to find the permutations of k items taken from set of n items. It should make use of the factorial function. The formula for permutation is given as

$$^nP_k = \frac{n!}{(n-k)!}$$

#include<iostream>
using namespace std;
int fact(int a){
if(a<=1) return 1;
else return a*fact(a-1);
}
int perm(int n,int k){
if(n<0 || k<0) return 0;
else if(n<k) return 0;
else return fact(n)/fact(n-k);
}
int main(){
int n,k,p;
cout<<"Enter n : ";
cin>>n;
cout<<"Enter k : ";
cin>>k;
p = perm(n,k);
cout<<"The permutations are "<<p;
}

Write the program to find the combinations. The formula for combination is given as

$$^nC_k = \frac{n!}{(n-k)!(k)!}$$