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IB DP Computer Science Study Notes

5.5.4 Sketching Binary Trees

Binary trees are pivotal in computer science, underpinning numerous algorithms and data structures. In this chapter, we delve into how to sketch binary trees and visualise key operations such as adding, deleting, and modifying nodes. Grasping these concepts through diagrammatic representation is crucial for comprehending the dynamic nature of tree structures.

Introduction to Binary Trees

A binary tree is a tree data structure in which each node has at most two children, typically differentiated as the left and right child. The top node of the tree is the root, and nodes without children are called leaves. Proficiency in sketching these trees is foundational for visualising their structure and operations.

Sketching Techniques for Binary Trees

Effective sketching of binary trees is essential for understanding their structure and relationships. Here's how you can do it:

  • Root Node: Begin with the root at the top. This represents the starting point of the tree.
  • Child Nodes: Draw the left and right children of each node, ensuring that each node branches into at most two children.
  • Left and Right Children: Position the left child to the left of the parent and the right child to the right, maintaining a clear distinction.
  • Leaves: Nodes at the bottom without any children are the leaves. They conclude the branches of the tree.

Visual representation is not just about structure but also about understanding the dynamic nature of binary trees.

Visualising Operations

Adding Nodes

  • Where to Add: The new node's position depends on certain rules. In binary search trees, for instance, this depends on the node's value compared to existing node values.
  • Sketching the Addition: Add the new node as a child, maintaining the binary tree properties.

Deleting Nodes

  • Deletion of Leaf Nodes: Removing a leaf node is straightforward as it doesn't have children.
  • Deletion of Nodes with Children: Requires reconnection of children to maintain tree integrity. In binary search trees, this often involves finding a suitable replacement from the node's subtrees.

Modifying Nodes

  • Changing Value: Directly change the node's data. This is simpler than adding or deleting as it doesn't impact the tree's structure.
  • Relocating Nodes: This is more complex as it may involve rebalancing the tree to maintain its properties.

Practical Examples

Example 1: Adding a Node

  • Scenario: Insert a node with value 6 into a binary search tree where the root is 10, with children 5 (left) and 12 (right).
  • Action: Since 6 is greater than 5 and less than 10, it is added as the right child of 5.

Example 2: Deleting a Node

  • Scenario: Remove a node with value 5, which has a left child 3 and no right child.
  • Action: Here, 3 can directly replace 5, maintaining the tree's structure.

Example 3: Modifying Node Data

  • Scenario: Change a node's value from 15 to 18 in a binary tree.
  • Action: Locate the node with 15 and change it to 18, keeping its position constant.

Diagrammatic Representation

Clarity is key in diagrammatic representation:

  • Nodes: Use circles or squares with numbers or letters indicating value.
  • Connections: Draw straight, unambiguous lines to denote relationships between parent and child nodes.
  • Spacing: Ensure that nodes and branches are spaced out evenly to avoid confusion.

Operations and Tree Balance

The operations of adding, deleting, and modifying nodes can affect a tree's balance:

  • Maintaining Balance: Post-operation, evaluate if the tree has become unbalanced. An unbalanced tree might necessitate reorganisation to optimise performance and operations.
  • Rebalancing Techniques: Methods like tree rotations (left or right) are often used to regain balance.

Advanced Topics in Binary Trees

Beyond the basics, binary trees encompass several complex operations and structures:

  • Tree Rotations: These are critical for maintaining balanced trees, especially in self-balancing binary trees like AVL trees.
  • Multi-level Operations: In larger trees, operations might have cascading effects, requiring adjustments at multiple levels.

Scenario-Based Exercises

Engaging with different scenarios enhances practical understanding:

Complex Scenario: Adding Nodes in AVL Tree

  • Given: An AVL tree that after adding a node, becomes unbalanced.
  • Task: Perform necessary rotations to balance the tree.
  • Solution: Identify the type of imbalance (left-left, left-right, right-right, right-left) and apply the appropriate rotation.

Concluding Thoughts

While this chapter provides a foundational understanding of sketching binary trees and visualising operations, the real mastery comes from consistent practice and application. Regularly sketching out different scenarios, and visualising the impact of various operations, deepens understanding and prepares you for more advanced topics in computer science.


Sketching binary trees is invaluable for understanding recursive operations, particularly depth-first traversals (inorder, preorder, and postorder). Traversal operations, by their nature, involve visiting each node of the tree in a specific order, and recursion is a natural method to achieve this in binary trees due to their hierarchical structure. By sketching, students can trace the path taken by the traversal algorithm, visually following the recursive call stack. For example, in an inorder traversal (left, root, right), sketching allows students to observe how the algorithm dives deep to the leftmost node, unwinds back to the root, and then proceeds to the right. This visual representation aids in understanding how recursion works, revealing the order in which nodes are visited and how the call stack evolves and resolves during the traversal process.

Sketching binary trees is pivotal in understanding and applying tree balancing techniques, such as rotations used in AVL and Red-Black Trees. Rotations are fundamental operations that adjust the structure of a tree to maintain its balance, thus ensuring optimal operation times for insertion, deletion, and search. By visually representing these rotations, one can observe how the tree’s shape and node levels change, directly affecting the tree’s balance. For instance, in an AVL tree, if sketching reveals that inserting a node has led to a left-left imbalance (a heavier left subtree), a right rotation on the imbalanced node can be visualised to restore balance. Sketching these rotations helps in both comprehending the mechanics of balance maintenance and in visualising the abstract concept of tree balance, making it easier to grasp the practical implications on tree performance.

Visualising node deletion in a binary tree, especially a binary search tree (BST), is crucial because the operation can be complex, involving several steps to maintain the BST properties. Sketching these steps aids in understanding the process and the tree's structural changes. When deleting a node, three scenarios arise: deleting a leaf node (simple case), a node with one child (requires reattachment of the child to the parent of the deleted node), or a node with two children (complex case, usually involving finding the in-order successor or predecessor to replace the deleted node). Sketching these scenarios helps grasp how different cases affect the tree’s structure and the need for potential rebalancing in self-balancing trees. Without a visual aid, understanding the intricate steps and their impact on the tree's properties and operations can be challenging.

Sketching binary trees is fundamental in understanding how binary search trees (BSTs) and balanced trees like AVL and Red-Black Trees contribute to efficient searching and sorting algorithms. By sketching, students can visualise the path followed during a search operation. For example, in a BST, searching for a value involves traversing from the root down to either the left or the right child, depending on whether the sought value is lesser or greater than the current node. This visualisation helps understand why searching in a BST is efficient, generally requiring O(log n) time, where n is the number of nodes. Similarly, while examining sorting algorithms like tree sort, sketching helps in understanding how elements are extracted in a sorted order through in-order traversal. This visual practice enhances comprehension of the underlying mechanics and efficiency of these algorithms.

Practice Questions

Given the initial binary tree structure:

Root = 8

Left Child of Root = 3 (which has left child = 1 and right child = 6)

Right Child of Root = 10

Add a node with value 4 to the tree and sketch the new tree structure. Explain your reasoning for the placement of the node.

The tree is a binary search tree (BST), where left children are less than their parent and right children are greater. The new node with value 4 must be placed in relation to the value 3 and its children (1 and 6). Since 4 is greater than 3 and less than 6, it should be added as the right child of 3. After the addition, the tree should show 4 as the right child of 3, maintaining the binary search tree properties.

Consider a binary tree where the root is 15, the left child of the root is 10, and the right child of the root is 20. If the node 10 has a left child 5 and a right child 12, describe how you would delete the node 10 and restructure the tree. Sketch and explain your final tree structure.

To delete node 10 from the binary tree, we must consider its children, 5 and 12. One approach is to replace node 10 with its immediate successor in in-order traversal, which in this case is 12. After removing 10, 12 becomes the new left child of the root (15), and 5 becomes the left child of 12. This reorganisation preserves the binary search tree properties: every node on the left of 15 is smaller, and those on the right are larger. The final tree will have 15 as the root, 12 as the left child of 15, 5 as the left child of 12, and 20 as the right child of 15.

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Written by: Alfie
Cambridge University - BA Maths

A Cambridge alumnus, Alfie is a qualified teacher, and specialises creating educational materials for Computer Science for high school students.

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