Functions and Loops

Complete function factorial in the following code snippet using

first a for-loop,
second a while-loop and
third with goto and if statements

Each variant should be equivalent to each other.

#include <stddef.h>
#include <stdio.h>

#define N 6

size_t factorial(size_t n)
{
    size_t res = 1;

    // your code here

    return res;
}

int main()
{
    printf("result = %zu\n", factorial(N));

    return 0;
}