Refactor in progress

kelvinlauKL · remlostime · commit f5c74b9aa9f3 · 2018-01-14T16:43:49.000-08:00
diff --git a/Encode and Decode Tree/readme.md b/Encode and Decode Tree/readme.md
@@ -1,27 +1,49 @@
 # Encode and Decode Binary Tree
 
-We need to design an algorithm to encode and decode a binary tree. 
-* **Encode**: Convert a tree into a string that can be stored in the disk.
-* **Decode**: Given the encoded string, you need to convert it into a Tree.
+> **Note**: The prerequisite for this article is an understanding of how [binary trees](https://github.com/raywenderlich/swift-algorithm-club/tree/master/Binary%20Tree) work.
 
-For example, you may serialize the following tree
+Trees are complex structures. Unlike linear collections such as arrays or linked lists, trees are *non-linear* and each element in a tree has positional information such as the *parent-child* relationship between nodes. When you want to send a tree structure to your backend, you need to send the data of each node, and a way to represent the parent-child relationship for each node.
 
+Your strategy in how you choose to represent this information is called your **encoding** strategy. The opposite of that - changing your encoded data back to its original form - is your **decoding** strategy. 
 
-as "[1,2,3,null,null,4,5]", just the same as how LeetCode OJ serializes a binary tree. You do not necessarily need to follow this format, so please be creative and come up with different approaches yourself.
-Note: Do not use class member/global/static variables to store states. Your serialize and deserialize algorithms should be stateless.
+There are many ways to encode a tree and decode a tree. The important thing to keep in mind is that encoding and decoding strategies are closely related. The way you choose to encode a tree directly affects how you might decode a tree. 
 
-## Solution
-For example, given a tree like this
->       a
->      /  \\
->
->    b     c
->
->    ​      /  \\
->    ​    d    e
+Encoding and decoding are synonyms to *serializing* and *deserializing* trees. 
 
-We can use inorder traversal to convert the tree into the string like this `a b # # c d # # e # #`
+As a reference, the following code represents the typical `Node` type of a binary tree:
 
-So, the idea is for the empty node, we use `#` to represent.
+```swift
+class BinaryNode<Element: Comparable> {
+  var data: Element
+  var leftChild: BinaryNode?
+  var rightChild: BinaryNode?
+
+  // ... (rest of the implementation)
+}
+```
+
+## Encoding
+
+As mentioned before, there are different ways to do encoding. For no particular reason, you'll opt for the following rules:
+
+1. The result of the encoding will be a `String` object.
+2. You'll encode using *pre-order* traversal.
+
+Here's an example of this operation in code:
+
+```swift
+extension BinaryNode {
+  var encodedString: String {
+    var str = ""
+    preOrderTraversal { str.append($0) }
+    return str
+  }
+  
+  func preOrderTraversal(visit: (T) -> ()) {
+    visit(data)
+    leftChild?.preOrderTraversal(visit: visit)
+    rightChild?.preOrderTraversal(visit: visit)
+  }
+}
+```
 
-For the decode process, we can still use inorder to convert the string back to a tree.
