What is the time complexity of searching in a binary search tree (BST)?

What is the time complexity of searching in a binary search tree (BST)?
4 min read
15 September 2023

Searching in a Binary Search Tree: Time Complexity Analysis and Algorithm Analysis

The Binary Search Tree is an important data structure used in computer science. It plays a key role in many algorithms and applications. The Binary Search Tree (BST) is a hierarchical structure with at most two nodes. It follows a particular ordering property. For any node, the left subtree has values that are less or equal to its value and the right subtree has values that are greater than its value. This property makes BST searching efficient.  Data Structures and Algorithms With Python Course in Pune

This comprehensive discussion will explore the time complexity involved in searching a BST. We will also examine the impact of the structure of the BST on search performance.

Binary search tree Basics:

Let’s review some basic BST properties before we begin the analysis.

  1. Root node: A BST’s topmost node is known as the root node. It is the beginning of any search operation.
  2. Child nodes: A BST has a maximum of two child nodes, the left and right child.
  3. Ordering property: This ordering property ensures that, for any node having a value of ‘V’, all nodes within its left subtree will have values lower or equal to V’, and all nodes within its right subtree will have values higher than V.

Searching a BST using

The BST search is based on its ordering property. The algorithm begins at the root when given a value to be searched for. It compares the target value ‘T” with the current node’s value. It proceeds to either the left or the right child depending on the comparison. This effectively reduces the search area by half each time. Data Structures and Algorithms With Python Classes in Pune

Here is the high-level search algorithm in a BST.

  1. Start by selecting the root node.
  2. The search is successful if the value of the current node equals “T”.
  3. If the value of ‘T’ (the current node) is less than ‘T’, then move the child to the left.
  4. If the value of ‘T” is greater than that of the current node, then move the child to the right.
  5. Repeat steps 2-4, until you find ‘T.’ Or reach a leaf-node (indicating that ‘T.’ is not present in the tree).

Time Complexity for BST Search:

The height of the BST tree is what determines how long it takes to search. The height of the tree will be logarithmic if the tree is balanced, meaning that each level is occupied. The time complexity for searching in a BST in this case is O(logN), where N represents the number of nodes.

The worst case scenario is when the BST has been skewed. This means that it is basically a linked-list. In this case the height of a tree equals the number of nodes N, resulting in an O(N) time complexity. A skewed BST violates its efficient search property.

Unbalanced vs. Balanced Trees:

Consider the following examples to illustrate the difference between the time complexity of a balanced tree and an unbalanced one:

Balanced tree (Optimal case):

Markdown 5 / \ 3 8 / \ / \ 1 4 7 9 

The height of this balanced BST is log(N), where N = 7. Searching for any value requires at most three comparisons.

Unbalanced tree (Worst case):

Markdown 1 \ 2 \ 3 \ 4 \ 5 

The height of this unbalanced BST is N (number nodes), so searching for a particular value involves N comparisons. The complexity of the time is O(N).  Data Structures and Algorithms With Python Training in Pune


Summary: The time complexity for searching in a Binary Search Tree is O(logN) when the tree balances and O(N), in the worst-case scenario (when it is not balanced). It is important to keep the BST balanced by using techniques such as AVL or Red-Black Trees. These self-balancing Binary Search Trees guarantee a logarithmic depth and, thus, a consistent O(logN) time complexity, regardless of input data order.

Understanding the time complexity BST search is critical for designing efficient algorithms in computer science applications such as databases, sorting algorithms, symbol tables and others.

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