斐波那契数列Python实现

def fbi(n):
    s1 = 1
    s2 = 1
    if(n<=0):
        return -1
    else:
        if( n==1 | n==2):
            return 1
        else:
            for num in range(2, n):  
                s2 = s1+s2
                s1 = s2-s1
            return s2


print(fbi(9))

时间复杂度为o(n)，空间复杂度为o(1)，用的是迭代法想加