Fibonacci Number.py

# LeetCode - 509 :

Time : O(n)
Space : O(1)

```    
class Solution:
    def fib(self, n: int) -> int:
        if n == 0:
            return 0
        if n == 1:
            return 1
        a = 0
        b = 1
        for i in range(n-1):
            a, b = b, a+b
        return b
```