递推

while True:
    try:
        n = int(input())
        total = []
        sum1 = 1 # 一个月兔子个数
        sum2 = 0 # 两个月兔子个数
        sum3 = 0 # 三个月以及三个月以上兔子个数
        total.append(sum1+sum2+sum3)
        for _ in range(1, n):
            sum3 += sum2 # 中兔子变大兔子
            sum2 = sum1 # 中兔子变大兔子
            sum1 = sum3 # 大兔子生小兔子
            total.append(sum1+sum2+sum3)
        print(total[-1])
    except:
        break

动态规划

while True:
    try:
        n = int(input())
        total = [0] * n
        for i in range(n):
            if i < 2:
                total[i] = 1
            else:
                total[i] = total[i-1] + total[i-2]
        print(total[-1])
    except:
        break