avl_tree.py: Add AVL Tree Data Structure

Closes #50

README.md

@@ -31,7 +31,7 @@ This repository contains examples of various algorithms written on different pro
 | [Queue](https://en.wikipedia.org/wiki/Queue_(abstract_data_type))                               |                                       | [:octocat:](queue/Cpp)                |                                       |                                       |
 | [Stack](https://en.wikipedia.org/wiki/Stack_(abstract_data_type))                               |                                       |                                       | [:octocat:](stack/Java)               | [:octocat:](stack/Python)             |
 | [Linear Linked List](https://en.wikipedia.org/wiki/Linked_list)                                 | [:octocat:](linked_list/C)            |                                       |                                       |                                       |
-| [AVL Tree](https://en.wikipedia.org/wiki/AVL_tree)                                              | [:octocat:](avl_tree/C)               |                                       | [:octocat:](avl_tree/Java)            |                                       |
+| [AVL Tree](https://en.wikipedia.org/wiki/AVL_tree)                                              | [:octocat:](avl_tree/C)               |                                       | [:octocat:](avl_tree/Java)            | [:octocat:](avl_tree/Python)          |
 
 
 ## Sample Run

avl_tree/Python/avl_tree.py

@@ -0,0 +1,134 @@
+#!/usr/bin/env python3
+
+
+class TreeNode:
+    """
+    A node class
+
+    :param val: Node value
+    """
+
+    def __init__(self, val):
+        self.val = val
+        self.left = None
+        self.right = None
+        self.height = 1
+
+
+class AVL_Tree:
+    """
+    AVL Tree class
+    """
+
+    def insert(self, root, key):
+        """
+        Recursive function to insert key in subtree rooted with node
+
+        :returns:   new root of subtree
+        """
+        # step 1 : normal BST
+        if not root:
+            return TreeNode(key)
+
+        if key < root.val:
+            root.left = self.insert(root.left, key)
+
+        else:
+            root.right = self.insert(root.right, key)
+        # Step 2 : Update the height of the ancestor root
+        root.height = 1 + max(self.getHeight(root.left),
+                              self.getHeight(root.right))
+        # Step 3 : get the balance factor
+        balance = self.getBalance(root)
+        # step 4 : if root is unbalanced, try the 4 cases
+        # case 1 - left left
+        if balance > 1 and key < root.left.val:
+            return self.rightRotate(root)
+        # Case 2 - Right Right
+        if balance < -1 and key > root.right.val:
+            return self.leftRotate(root)
+        # Case 3 - Left Right
+        if balance > 1 and key > root.left.val:
+            root.left = self.leftRotate(root.left)
+            return self.rightRotate(root)
+        # Case 4 - Right Left
+        if balance < -1 and key < root.right.val:
+            root.right = self.rightRotate(root.right)
+            return self.leftRotate(root)
+
+        return root
+
+    def leftRotate(self, z):
+        y = z.right
+        T2 = y.left
+        # Perform rotation
+        y.left = z
+        z.right = T2
+        # Update heights
+        z.height = 1 + max(self.getHeight(z.left), self.getHeight(z.right))
+        y.height = 1 + max(self.getHeight(y.left), self.getHeight(y.right))
+        # Return the new root
+        return y
+
+    def rightRotate(self, z):
+        y = z.left
+        T3 = y.right
+        # Perform rotation
+        y.right = z
+        z.left = T3
+        # Update heights
+        z.height = 1 + max(self.getHeight(z.left), self.getHeight(z.right))
+        y.height = 1 + max(self.getHeight(y.left), self.getHeight(y.right))
+        # Return the new root
+        return y
+
+    def getHeight(self, root):
+        if not root:
+            return 0
+
+        return root.height
+
+    def getBalance(self, root):
+        if not root:
+            return 0
+
+        return self.getHeight(root.left) - self.getHeight(root.right)
+
+    def preOrder(self, root):
+        if not root:
+            return
+
+        print("{0} ".format(root.val), end="")
+        self.preOrder(root.left)
+        self.preOrder(root.right)
+
+    def inOrder(self, root):
+        if not root:
+            return
+
+        self.inOrder(root.left)
+        print("{0} ".format(root.val), end="")
+        self.inOrder(root.right)
+
+
+def main():
+    myTree = AVL_Tree()
+    root = None
+
+    root = myTree.insert(root, 10)
+    root = myTree.insert(root, 20)
+    root = myTree.insert(root, 30)
+    root = myTree.insert(root, 40)
+    root = myTree.insert(root, 50)
+    root = myTree.insert(root, 28)
+
+    print("Pre-order Traversal of the constructed Tree: ")
+    myTree.preOrder(root)
+    print()
+    print("In-order Traversal of the constructed Tree: ")
+    myTree.inOrder(root)
+    print()
+
+
+if __name__ == "__main__":
+    main()