tree traversal examples update readme

Author: ivanmmarkovic

README.md

@@ -79,7 +79,10 @@ but it also contains other data structures, algorithms and problems.
 - [Tree traversal](https://github.com/ivanmmarkovic/Problem-Solving-with-Algorithms-and-Data-Structures-using-Python/tree/master/trees/tree-traversal)
     - [Methods](https://github.com/ivanmmarkovic/Problem-Solving-with-Algorithms-and-Data-Structures-using-Python/blob/master/trees/tree-traversal/treenode.py)
     - [Functions](https://github.com/ivanmmarkovic/Problem-Solving-with-Algorithms-and-Data-Structures-using-Python/blob/master/trees/tree-traversal/functions.py)
-
+    - [Preorder traversal example](https://github.com/ivanmmarkovic/Problem-Solving-with-Algorithms-and-Data-Structures-using-Python/blob/master/trees/tree-traversal/preorder-traversal-example.py)
+    - [Inorder traversal example](https://github.com/ivanmmarkovic/Problem-Solving-with-Algorithms-and-Data-Structures-using-Python/blob/master/trees/tree-traversal/inorder-traversal-example.py)
+    - [Postorder traversal example](https://github.com/ivanmmarkovic/Problem-Solving-with-Algorithms-and-Data-Structures-using-Python/blob/master/trees/tree-traversal/postorder-traversal-example.py)
+    
 ### [Sorting](https://github.com/ivanmmarkovic/Problem-Solving-with-Algorithms-and-Data-Structures-using-Python/tree/master/sorting)
 - [Bubble sort](https://github.com/ivanmmarkovic/Problem-Solving-with-Algorithms-and-Data-Structures-using-Python/blob/master/sorting/bubble-sort.py)
 - [Short bubble](https://github.com/ivanmmarkovic/Problem-Solving-with-Algorithms-and-Data-Structures-using-Python/blob/master/sorting/short-bubble.py)

trees/tree-traversal/preorder-traversal-example.py

@@ -40,7 +40,7 @@ def tree_parser(s: str) -> TreeNode:
 tree_node: TreeNode = tree_parser(string)
 
 
-def preorder(node: TreeNode, space: int = 0) -> str:
+def preorder(node: TreeNode, space: int = 0):
     if node is not None:
         print(" " * space, node.key)
         preorder(node.get_left_child(), space + 2)