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IB DP Computer Science Study Notes

5.5.2 Terminology in Binary Trees

In the realm of computer science, particularly for Higher Level (HL) students, comprehending the structure and terminology of binary trees is essential. This segment explores these terminologies, their specific roles, and how they contribute to the hierarchical structure of binary trees.

Introduction to Binary Trees

A binary tree is a pivotal data structure in computer science where each node has, at most, two children. This structure, characterised by its left and right children, enables a wide range of efficient computational operations including insertion, deletion, searching, and traversing.

Core Terminologies in Binary Trees


  • Definition: In binary trees, a parent is a node that extends to one or two other nodes, known as children.
  • Role:
    • A pivotal node connecting children in the tree.
    • Determines the depth and structure of the tree, impacting functions like balancing and searching.

Left-Child and Right-Child

  • Definition: The left and right children are the two possible extensions of a parent node in a binary tree.
  • Roles:
    • Left-Child: Positioned to the left of the parent, playing a critical role in tree traversal and balancing algorithms.
    • Right-Child: Found on the right side of its parent, equally important in structuring and operating the tree.


  • Definition: This is the uppermost node in a binary tree from which all nodes descend.
  • Role:
    • The primary access point for any operation in the tree.
    • Dictates the tree's height and overall structure.


  • Definition: Nodes without children, residing at the bottom of the tree.
  • Role:
    • Marks the completion of a tree path.
    • Essential in determining the tree's depth and in operations like pruning.


  • Definition: A node in a binary tree, along with its descendants, forms a subtree, constituting a distinct tree within the larger structure.
  • Role:
    • Enhances modular understanding and manipulation of binary trees.
    • Plays a key role in recursive algorithms and problem-solving within trees.

Hierarchical Structure and Implications

The structure of binary trees is inherently hierarchical, organising data in a manner that reflects their relational properties.

  • Root at the Top: As the originator of all nodes, the root is at the apex of the hierarchy.
  • Levels Descending from the Root: Each subsequent level represents a deeper branch in the tree, encompassing the children of the previous level.
  • Leaves at the Bottom: Concluding the hierarchy, leaves are the terminal nodes.

Hierarchical Relationships

  • The binary tree's functionality and its operations rely heavily on parent-child relationships. The placement of nodes as left or right children impacts the tree's physical representation and navigational logic.
  • Searching: Initiating from the root, the search operation considers the left or right placement to move through the tree efficiently.
  • Insertion and Deletion: These are core operations requiring keen awareness of hierarchical relations to preserve the structural and operational integrity of the tree.
  • Traversal: The hierarchical structure of the tree influences traversal paths. Inorder, preorder, and postorder traversals navigate the tree in distinct patterns, leveraging the parent-child relationships.

Practical Application in Algorithms


  • Binary trees expedite search operations. Starting at the root, the algorithm compares the search value and decides to traverse left or right, efficiently narrowing down the search space.

Insertion and Deletion

  • For insertion, the new node must find its correct position following the binary tree rules — smaller values to the left and larger to the right, ensuring the tree remains ordered.
  • Deletion involves three cases: removing a leaf, a node with one child, or with two children. Each scenario requires careful manipulation of parent-child links to maintain tree integrity.


  • Inorder (Left, Root, Right): Reveals the nodes in ascending order for binary search trees.
  • Preorder (Root, Left, Right): Useful for copying the tree or prefix expression evaluation.
  • Postorder (Left, Right, Root): Employed in postfix expression evaluation and certain tree deletion scenarios.


Understanding binary tree terminology not only underpins theoretical knowledge but also practical application in algorithms, data structure manipulation, and systems design. Mastering these concepts is fundamental for IB Computer Science HL students, paving the way for advanced study in more complex structures and algorithmic strategies.


While both binary trees and binary search trees (BSTs) are fundamental data structures with a maximum of two children per node, the key difference lies in how the nodes are organised. A binary tree is a broader concept with no specific ordering required among the nodes. In contrast, a BST is a type of binary tree that follows a specific order: for any given node, all elements in its left subtree are lesser, and those in the right subtree are greater. This property of BSTs makes them extremely efficient for search operations, providing a balanced approach between linked lists and sorted arrays.

Balance in a binary tree significantly impacts its operations, particularly in terms of efficiency. A balanced tree ensures that the nodes are distributed evenly, so the depth of the tree is kept to a minimum. This balancing reduces the worst-case complexity of operations like search, insertion, and deletion, which in a balanced binary tree remain closer to O(log n) where n is the number of nodes. In an unbalanced tree, these operations can degrade to O(n), particularly in skewed trees where the structure resembles a linked list. Thus, maintaining balance is essential for efficient operation of binary trees, especially in large datasets.

Binary trees are preferred over other data structures like arrays or linked lists in scenarios requiring hierarchical organisation of data with efficient insertions, deletions, and searches. Due to their structure, binary trees (especially when balanced) can offer better performance for these operations than arrays or linked lists. For instance, searching in a balanced binary tree has a complexity of O(log n), compared to O(n) in a linked list and potentially O(log n) in sorted arrays (but with slower insertions and deletions due to shifting elements). Additionally, binary trees are favoured in applications like implementing priority queues, BST-based map and set implementations, and representing sorted data, allowing quick traversal (inorder, preorder, postorder), which isn't as straightforward in arrays or linked lists.

A binary tree cannot have more than one root. By definition, a binary tree is a hierarchical structure starting from a single topmost node, known as the root, from which all other nodes (children) descend. The uniqueness of the root is crucial for maintaining the tree's hierarchical and organised structure. Having more than one root would violate the fundamental properties of a binary tree, leading to ambiguity in parent-child relationships and a breakdown in the tree's logical structure, potentially turning it into a forest (a collection of trees) rather than a single tree.

The depth of a binary tree is significant as it directly influences the efficiency of various operations like search, insert, and delete. The depth is defined as the number of edges in the longest path from the root node to a leaf. It is calculated by starting at the root (depth 0) and incrementing the count for each level traversed downwards to the leaf. A shallow tree (with minimal depth) leads to faster operations, as fewer steps are required on average to reach any node from the root. Conversely, a deep tree might suffer in performance, mirroring issues similar to a linear data structure, such as a linked list, where traversal time increases with the number of elements.

Practice Questions

Given a binary tree with the values 15 as the root, 10 as the left child of the root, and 20 as the right child of the root, identify the root, each leaf, and explain the reason behind your identification.

The root of the given binary tree is the node with the value 15, as it is the topmost node from which all other nodes originate. The leaves in this binary tree are the nodes with values 10 and 20. This identification is based on the fact that leaves in a binary tree are nodes without any children. In the given tree, both nodes 10 and 20 do not have any further extensions or child nodes branching out from them, thereby qualifying them as leaves.

Describe the process and outcome of inserting the value 5 into the previously mentioned binary tree, following standard binary tree insertion rules.

To insert the value 5 into the binary tree, we start at the root, comparing the value to be inserted with the current node value. Since 5 is less than the root value of 15, we move to the left child. Comparing 5 with 10, we find 5 is again smaller, and since 10 has no left child, 5 is inserted as the left child of 10. The tree now has a new leaf, 5, maintaining the binary tree's property where all left children are smaller than their parent nodes, and all right children are larger.

Alfie avatar
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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