題目如下：

The Fibonacci numbers, commonly denoted F(n) form a sequence, called the Fibonacci sequence, such that each number is the sum of the two preceding ones, starting from 0 and 1. That is,

F(0) = 0,   F(1) = 1
F(N) = F(N - 1) + F(N - 2), for N > 1.

Given N, calculate F(N).

Example 1:

Input: 2
Output: 1
Explanation: F(2) = F(1) + F(0) = 1 + 0 = 1.

Example 2:

Input: 3
Output: 2
Explanation: F(3) = F(2) + F(1) = 1 + 1 = 2.

Example 3:

Input: 4
Output: 3
Explanation: F(4) = F(3) + F(2) = 2 + 1 = 3.

Note:

0 ≤ N ≤ 30.

解題思路：遞歸的經典例子，當然為了提高效率，可以緩存中間結果。這題好像以前是Premium的，最近解鎖了。

代碼如下：

class Solution(object):
    def fib(self, N):
        """
        :type N: int
        :rtype: int
        
 
"""
        if N == 0 or N == 1:
            return N
        return self.fib(N-1) + self.fib(N-2)
        

【leetcode】509. Fibonacci Number