Method factorial

60. Permutation Sequence/src/main.rs:23–28

used for small num a: a!

(n: usize)

    /// used for small num a: a!
    pub fn factorial(n: usize) -> usize {
        match n {
            0 | 1 => 1,
            _ => n * Self::factorial(n - 1),
        }
    }
}

#[cfg(test)]