$f(n) = \begin{cases} 1 & \text{if } n \leq 1 \\ n \cdot f(n-1) & \text{if } n > 1 \end{cases}$

$f(n, \text{acc}) = \begin{cases} \text{acc} & \text{if } n \leq 1 \\ f(n-1, n \cdot \text{acc}) & \text{if } n > 1 \end{cases}$

$f(n) = \begin{cases} 0 & \text{if } n = 0 \\ f(n-1) + n & \text{if } n > 0 \end{cases}$

All of the above