509. Fibonacci Number-斐波那契数

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.

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

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

Constraints:
0 <= n <= 30

Solution in python:

class Solution:
    def fib(self, n: int) -> int:
        n0 = 0
        n1 = 1
        if n <= 1:
            return n
        while n >= 2:
            n0, n1 = n1, n0+n1
            n -= 1
        return n1