题解 | #兔子的数量#

import sys

mon = sys.stdin
mon = int(str(mon.read()))
out = 0


def count_rabbit(mont):
    t_out1 = 0
    t_out2 = 0
    t_out = 0
    for _ in range(1, mont + 1):
        if _ == 1:
            t_out1 = 1
            t_out2 = 1
        else:
            tmp = t_out1
            t_out1 = t_out2
            t_out2 = tmp + t_out1
        t_out = t_out1 + t_out2
    return t_out

out = count_rabbit(mon)
print(out)

解题思路在于梳理两个加数之间的关系，当n=1的时候 可以看成a=1 b=1 c=a+b; n=2的时候，a = 之前的b b =之前a+现在的a，c = 现在的a + 现在的b， 以此类推。