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Day 3: Recursive Fibonacci Series

Learning Goals

• Solve algorithm problems using recursion

Instructions

Find the nth element in the Fibonacci series. The Fibonacci sequence starts with a 0 followed by a 1. After that, every value is the sum of the two values preceding it. Here are the first seven values as an example: 0, 1, 1, 2, 3, 5, 8.

Input: 0
Output: 0

Input: 2
Output: 1

Input: 10
Output: 55

If you solved this problem before iteratively, you may wish to convert that solution to a recursive version. Here are two iterative solutions - one in Ruby and one in JS:

def fibonacci(n)
  return n if n < 2

  values = [0, 1]

  (n - 1).times do
    values << values[-1] + values[-2]
  end

  values.last
end

function fibonacci(n) {
  if (n < 2) {
    return n;
  }

  const values = [0, 1];

  for (let i = 0; i < n - 1; ++i) {
    values.push(values[values.length - 1] + values[values.length - 2]);
  }

  return values[values.length - 1];
}

Stuck? Here are some hints:

• Code the base cases first.
• You may wish to look up how the fibonacci sequence is expressed as a formula.
• Start small. What needs to happen if n is 1 or n is 2?

Use the language of your choosing. We've included starter files for some languages where you can pseudocode, explain your solution and code.

Before you start coding

1. Rewrite the problem in your own words
2. Validate that you understand the problem
3. Write your own test cases
4. Pseudocode
5. Code!

And remember, don't run our tests until you've passed your own!

How to run your own tests

Ruby

1. cd into the ruby folder
2. ruby <filename>.rb

JavaScript

1. cd into the javascript folder
2. node <filename>.js

How to run our tests

Ruby

1. cd into the ruby folder
2. bundle install
3. rspec

JavaScript

1. cd into the javascript folder
2. npm i
3. npm test