Lecture 17

Lecture 17, Tue 11/28

Binary Search Trees cont.

Recorded Lecture: 11_28_23

Link to previous F21 in-person Final: F21 Sample Final
- Note: use this as a supplemental study guide - the content and difficulty will vary.

Binary Search Tree (BST)

Recall that a binary tree is a tree structure where a node may have at most two children
Binary Search Trees (BST) are binary trees that have the following property:
- Values that are less than the parent are found in the left subtree
- Values that are greater than the parent are found in the right subtree
- This is known as the BST property
Binary Search Trees are also one way to implement a Map Abstract Data Type
- A Map ADT maps keys to corresponding values
- Think of keys defining where in the BST structure a node gets inserted
- And each node has a corresponding value field
- Similar to how Python Dictionaries work on a high-level (but the underlying implementation between a Python Dictionary and BST are different (each with pros / cons))
Example: Inserting the following keys into a BST

Note: Insertion order of elements affects the structure of the tree!
Example: Inserting the same elements in different order

Also note: inorder traversal in a BST will visit the nodes in order - try it out!

BST TreeNode and BST Implementation

# TreeNode.py

class TreeNode:
	def __init__(self,key,val,left=None,right=None, parent=None):
		self.key = key
		self.payload = val
		self.leftChild = left
		self.rightChild = right
		self.parent = parent

	def hasLeftChild(self):
		return self.leftChild # Note: Python considers None as a False value

	def hasRightChild(self):
		return self.rightChild

	def isLeftChild(self):
		return self.parent and self.parent.leftChild == self

	def isRightChild(self):
		return self.parent and self.parent.rightChild == self

	def isRoot(self):
		return not self.parent

	def isLeaf(self):
		return not (self.rightChild or self.leftChild)

	def hasAnyChildren(self):
		return self.rightChild or self.leftChild

	def hasBothChildren(self):
		return self.rightChild and self.leftChild

	def replaceNodeData(self,key,value,lc,rc):
		self.key = key
		self.payload = value
		self.leftChild = lc
		self.rightChild = rc
		if self.hasLeftChild():
			self.leftChild.parent = self
		if self.hasRightChild():
			self.rightChild.parent = self

# BinarySearchTree.py

from TreeNode import TreeNode

class BinarySearchTree:
	def __init__(self):
		self.root = None # A BST just needs a reference to the root node
		self.size = 0 # Keeps track of number of nodes

	def length(self):
		return self.size

	def put(self,key,val):
		if self.root:
			self._put(key,val,self.root)
		else:
			self.root = TreeNode(key,val)
		self.size = self.size + 1

	# helper method to recursively walk down the tree
	def _put(self,key,val,currentNode):
		if key < currentNode.key:
			if currentNode.hasLeftChild():
				self._put(key,val,currentNode.leftChild)
			else:
				currentNode.leftChild = \
				TreeNode(key,val,parent=currentNode)
		else:
			if currentNode.hasRightChild():
				self._put(key,val,currentNode.rightChild)
			else:
				currentNode.rightChild = \ 
				TreeNode(key,val,parent=currentNode)


	def get(self,key): # returns payload for key if it exists
		if self.root:
			res = self._get(key,self.root)
			if res:
				return res.payload
			else:
				return None
		else:
			return None

	# helper method to recursively walk down the tree
	def _get(self,key,currentNode): 
		if not currentNode:
			return None
		elif currentNode.key == key:
			return currentNode
		elif key < currentNode.key:
			return self._get(key,currentNode.leftChild)
		else:
			return self._get(key,currentNode.rightChild)

# pytests

from BinarySearchTree import BinarySearchTree

def test_constructBST():
	BST = BinarySearchTree()
	assert BST.root == None
	assert BST.length() == 0

def test_insertRoot():
	BST = BinarySearchTree()
	BST.put(10, "ten")
	assert BST.root.key == 10
	assert BST.root.payload == "ten"
	assert BST.root.hasLeftChild() == None
	assert BST.root.hasRightChild() == None
	assert BST.root.isLeftChild() == None
	assert BST.root.isRightChild() == None
	assert BST.root.isRoot() == True
	assert BST.root.hasAnyChildren() == None
	assert BST.root.isLeaf() == True
	assert BST.root.hasBothChildren() == None
	BST.root.replaceNodeData(20, "twenty", None, None)
	assert BST.root.key == 20
	assert BST.root.payload == "twenty"

def test_insertNodes():
	BST = BinarySearchTree()
	BST.put(10, "ten")
	BST.put(20, "twenty")
	BST.put(15, "fifteen")
	BST.put(5, "five")
	assert BST.root.key == 10
	assert BST.root.leftChild.key == 5
	assert BST.root.rightChild.key == 20
	assert BST.root.rightChild.leftChild.key == 15