递归和迭代计算过程
f(n) = n; n<3
f(n) = f(n-1) + 2*f(n-2) + 3*f(n-3); n>=3
递归
1 ;; Scheme code
2 (define (foo1 n)
3 (cond
4 ((< n 3) n)
5 (else (+
6 (foo1 (- n 1))
7 (* 2 (foo1 (- n 2)))
8 (* 3 (foo1 (- n 3)))))))
迭代
1 ;; Scheme code
2 (define (foo2 n)
3 (local ((define (f a b c counter)
4 (cond
5 ((= counter 2) c)
6 (else (f b c (+
7 (* 3 a)
8 (* 2 b)
9 c) (- counter 1))))))
10 (cond
11 ((< n 3) n)
12 (else (f 0 1 2 n)))))
