# [LeetCode]#509. Fibonacci Number

**Environment: Python 3.8**

**Key technique: Recursive**

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:** n = 2

**Output:** 1

**Explanation:** F(2) = F(1) + F(0) = 1 + 0 = 1.

**Analysis:**

- If n=0 and n=1, return n.
- If n>1, use recursive method and return f(n-1)+f(n-2)

**Solution:**

`class Solution:`

def fib(self, n):

if n==0 or n==1:

return n

else:

return self.fib(n-1)+self.fib(n-2)

**Submissions:**

**Reference:**

https://openhome.cc/Gossip/AlgorithmGossip/FibonacciNumber.htm