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Trees: Python

Our implementation of tree traversal has a slight hiccup. Trees grow many levels deep, but we've only accounted for one parent-child relationship.

How is this a problem?

```py
root = TreeNode('Founder')
child_a = TreeNode('VP of Bananas')
child_b = TreeNode('Executive Assistant')
child_c = TreeNode('Banana R & D')

# adding children to the root
root.add_child(child_a)
root.add_child(child_b)

# assigning child_c to child_a creates an additional level in the tree
child_a.add_child(child_c)

root.traverse()
# prints "Founder", "VP of Bananas", "Executive Assistant"
```

"VP of Bananas" is a child to "Founder", **and** a parent to "Banana R & D". `.traverse()` only goes one level deep which leaves out "Banana R & D". Pull on your gardening gloves and let's fix that!



Tree Implementation V: Traversing Root to Leaf

Let's start by defining our `TreeNode` class. We'll begin with having our node store a value, and additional functionality can be layered on in the following exercises.

Tree Implementation I: Planting Seeds

Before we start building (planting?) our trees, let's do a quick inventory of what we'll need in our Python implementation. We're going to make the class `TreeNodes`.

`TreeNodes`:
- have a value
- have a reference to zero or more other `TreeNodes`
- can add a node as a child
- can remove a child 
- can _traverse_ (or travel through) connected nodes

Trees Introduction

Trees are an abstract idea that we're making concrete in Python. When implementing these abstract data structures, it's important to leverage the features of your language. 

Let's refactor `.remove_child()` to use Python's list comprehension. As a quick refresher on list comprehension:

```py
nums = [1, 2, 3, 4, 5]
evens = [num for num in nums if num % 2 == 0]
# [2, 4]
```

Tree Implementation IIIa: Tuning the Pruning

Our implementation has covered adding and removing nodes. Let's expand the functionality and add the ability to move through connected nodes. 

Tree traversal is a standard operation for finding nodes with a specific value or printing all the nodes available in a tree. 

We'd like to do the following:

```py
root = TreeNode('Founder')
child_a = TreeNode('VP of Bananas')
child_b = TreeNode('Executive Assistant')

root.add_child(child_a)
root.add_child(child_b)

root.traverse()
# prints "Founder", "VP of Bananas", "Executive Assistant"

```



Tree Implementation IV: Traversing

Congratulations, you have implemented a tree in Python.

For review, in our implementation:
- Trees are a Python class called `TreeNode`.
- A `TreeNode` has two properties, `value` and `children`.
- Nodes hold any type of data inside `value`.
- `children` is a list, which can be empty or hold other instances of `TreeNode`.
- We add to `children` by using the list method `.append`.
- We remove from `children` by filtering the list.

Tree Review

We have a working `TreeNode` class, but there's no time to enjoy a refreshing glass of lemonade. Trees are all about data hierarchy, and we need a parent-child relationship to make that work.

To review: child nodes are held as references by another instance of `TreeNode`, known as the parent node. 

```py
parent = TreeNode('CEO')
child = TreeNode('Executive Assistant')
print(parent.children)
# []
parent.add_child(child)
print(parent.children)
# [child]
```

We'll store the references to child nodes in a Python list and define an `add_child` method on our `TreeNode` class which will add nodes to that list.



Tree Implementation II: Think of the Children

Let's explore how to remove nodes from a tree. Remember, child nodes are held in a list within the parent node. To remove a child, we need to remove that node from the list.

We want the following functionality:

```py
print(root.children)
# [child_a, child_b, child_c]
root.remove_child(child_b)
print(root.children)
# [child_a, child_c]
```

* Call `.remove_child` on a specific node.
* Pass another node as an argument
* Remove from `.children` any nodes which match the argument node.

