对于简洁性，设F（x）为递归Fibonacci

For terseness, Let F(x) be the recursive Fibonacci

F(10) = F(9)                      + F(8)
F(10) = F(8)        + F(7)        + F(7) + F(6)
F(10) = F(7) + F(6) + F(6) + F(5) + 4 more calls.
....

所以你要两次拨打F（8），
F（7）3次，F（6）5次，F（5）7次..依此类推

So your are calling F(8) twice, F(7) 3 times, F(6) 5 times, F(5) 7 times.. and so on

因此，输入越大，树变大更大。

So with larger inputs, the tree gets bigger and bigger.