题解 | 牛牛学数列6

n=int(input())
A1,A2,A3=0,1,1
An=0
if n==1:
    An=0
elif n==2 or n==3:
    An=1
else:
    for i in range(4,n+1):
        An=A1+2*A2+A3
        A1,A2=A2,A3
        A3=An
    print(An)