fibonacci_number.py

# 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.

class Solution:
    def fib(self, n: int) -> int:
        # helper recursive function to calculate the sequence
        def help_fib(n):
            if n == 0:
                return 0
            elif n == 1:
                return 1
            else:
                return help_fib(n-1) + help_fib(n-2)
    
        # main
        return help_fib(n)