factorial.cpp

#include <cstdio>
#include <functional>
using namespace std;

int edgeOperation(int &n);
int productOperation(int &n);
const function<int(int &)> operations[2] = {&productOperation, &edgeOperation};

int edgeOperation(int &n) {
    --n;
    return (n + 1) * operations[!!n](n);
}

int productOperation(int &n) {
    return 1;
}

int factorial(int n) {
    return operations[!!n](n);
}

int main() {
    printf("%d\n", factorial(10));
    return 0;
}