Fibonacci Number

class Solution(object):
    def fib(self, n):
        """
        :type n: int
        :rtype: int
        """
        '''def brute_force(n):
            if n == 0:
                return 0
            if n == 1:
                return 1
            if n>1:
                return brute_force(n-1)+brute_force(n-2)
            
        out1 = brute_force(n)
        return out1
        
        cache = [0]*(n+1)
        
        def memoization(n):
            if n == 0:
                return 0
            if n == 1:
                return 1
            
            cache[0] = 0
            cache[1] = 1
            cache[2] = 1
            
            if cache[n]==0:
                cache[n]=memoization(n-1)+memoization(n-2)
            
            return cache[n]
        
        out2 = memoization(n)
        return out2
        
        cache = [0]*(n+1)
        def top_down(n):
            if n == 0:
                return 0
            if n == 1:
                return 1
            
            cache[0] = 0
            cache[1] = 1
            cache[2] = 1
            
            for i in range(3, n+1):
                cache[i]=cache[i-1]+cache[i-2]
                
            return cache[n]
        
        out3 = top_down(n)
        return out3'''
        
        def bottom_up(n):
            if n == 0:
                return 0
            if n == 1:
                return 1
            
            sum_num, first, second = 1, 0, 1
            
            for i in range(2, n+1):
                sum_num = first+second
                first = second
                second = sum_num
                
            return sum_num
        
        out4 = bottom_up(n)
        return out4