斐波那契数列

1，1，2，3，5，8.....

 F(n) = F(n-1) + F(n-2)
 F(0) = 1  F(1) = 1

def fibnacci(n):
    if n == 0 or n == 1:
        return 1
    else:
        return fibnacci(n-1) + fibnacci(n-2)

def fib2(n):
    li = [1,1]
    for i in range(2, n+1):
        li.append(li[-1]+li[-2])
    return li[n]

def fib3(n):
    a = 1
    b = 1
    c = 1
    for i in range(2, n+1):
        c = a + b
        a = b
        b = c
    return c