"""

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(object):
    def fib(self, n):
        """
        :type n: int
        :rtype: int
        """
        
        if n <= 1: return n
        if n == 2: return 2
        
        current = 0
        prev1 = 1
        prev2 = 1
        
        for i range(3, n+1):
            current = prev1 + prev2
            prev1 = prev2
            prev2 = current
            
        return current
        
        # just to switch the value
      
        # recursion is another way, but take longer time but less space
        
        


