f(n) = f(n-1) + f(n-2)+f(n-3)
f(1)=1
f(2)=2
f(3)=4
f(4)=7
f(5)=13
f(6)=24
f(7)=44
f(8)=81
f(9) = 149
f(10) = 274
f(11) = 504
f(12) = 927
f(13)=1705
f(14)=3136
f(15)=5768
如果思想差不多.大概是这样吧.等待高手.还可以一步上三级