bebepeng / recursion-ruby

Intro to Recursion

Background

Recursion is a technique where a method can call itself. Recursion can be used to traverse tree structures.
For example, take the following tree, where each letter represents a node in the tree:

          A
      ____|____
      |       |
      B       C
    __|__   __|__
    |   |   |   |
    D   E   F   G
                |
                H

In this tree, we can say that

• A is the "root" node
• A has two child nodes: [B, C]
• B has two child nodes: [D, E]
• C has two child nodes: [F, G]
• G has one child node: [H]

Let's say you wanted to go over the entire tree and print the letters out, you could write code like this:

puts root.letter                                      # prints "A"
root.child_nodes.each do |child_node_1|               # goes over ["B", "C"]
  puts child_node_1.name
  child_node_1.child_nodes.each do |child_node_2|     # goes over ["D", "E"], then ["F", "G"]
    puts child_node_2.name
    child_node_2.child_nodes.each do |child_node_3|   # goes over ["H"]
      puts child_node_3.name
    end
  end
end

You can see that this only works if the tree has 4 levels. But what if it had 8, or 100 levels? Basically that code is doing the same thing, but from a different starting point:

1. Start with A, print A's letter
2. Go over all of A's children and repeat step 1, but start with B, then with C

We could write that in Ruby like so:

def print_letters(node)
  puts node.name
  node.child_nodes.each do |child_node|
    print_letters(child_node)
  end
end

Challenge

Your mission, should you choose to accept it, is to make node_walker_spec.rb pass.

Setup

• Fork
• Clone
• Turn on TravisCI for the fork by visiting https://travis-ci.org/profile/, clicking the "Sync now" button and scrolling down to find the repository to build.
• Create a new branch for your work using git checkout -b v1
• Implement specs and code
• Push using git push -u origin v1

Further Practice

This warmup can be completed multiple times to increase your comfort level with the material. To work on this from scratch, you can:

1. Add an upstream remote that points to the original repo git remote add upstream [e-mail]:gSchool/THIS-REPO.git
2. Fetch the latest from the upstream remote using git fetch upstream
3. Create a new branch from the master branch of the upstream remote git checkout -b v2 upstream/master
4. Implement specs and code
5. Push using git push -u origin v2

Each time you do the exercise, create a new branch. For example the 3rd time you do the exercise the branch name will be v3 instead of v2.