509. Fibonacci Number. Sol

still Easy The subject of , So do more to improve self-confidence （bushi）

Recursion is simple , But this is not done with recursion , Is to use ordinary iteration , Just one for Circulation is enough

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

  class Solution:
      def fib(self, n):
          if n==0 or n==1:
              return n
          else:
              prev1 = self.fib(0)
              prev2 = self.fib(1)
              for _ in range(n-1):
                  temp = prev2
                  prev2 = prev1 + prev2
                  prev1 = temp
              return prev2