計算整數的階乘

實現遞迴函式的一種簡單演算法是階乘。

;;Compute the factorial for any n >= 0. Precondition: n >= 0, n is an integer.
(defun factorial (n)
    (cond
        ((= n 0) 1) ;; Special case, 0! = 1
        ((= n 1) 1) ;; Base case, 1! = 1
        (t
            ;; Recursive case
            ;; Multiply n by the factorial of n - 1.
            (* n (factorial (- n 1)))
        )
    )
)