Introduction to Binary Search Trees

Introduction to Binary Search Trees

A Binary Search Tree (BST) is a binary tree in which each node has a value, and for every node, the values in the left subtree are less than the node's value, and the values in the right subtree are greater than the node's value. BSTs are used in various applications such as searching, sorting, and maintaining a dynamic set of ordered elements.

Simple Example

Consider a BST with the following structure:

    /  \
   5   20
  / \   / \
 3   7 15  25

In this BST, the root node is 10. The left subtree of 10 contains values less than 10, and the right subtree contains values greater than 10. This property holds for every node in the tree.

Properties of Binary Search Trees

Binary Search Trees have several important properties:

  • Search Efficiency: BSTs allow for efficient searching, insertion, and deletion operations, with average time complexity of O(log n).
  • In-order Traversal: An in-order traversal of a BST visits the nodes in ascending order of their values.
  • Dynamic Set Operations: BSTs support dynamic set operations such as insertion, deletion, and membership tests efficiently.
  • Balance: The performance of a BST is optimal when it is balanced, meaning the heights of the left and right subtrees of any node differ by at most one.

Binary Search Tree Implementation

class Node:
    def __init__(self, key):
        self.left = None
        self.right = None
        self.val = key

class BinarySearchTree:
    def __init__(self):
        self.root = None

    def insert(self, key):
        if self.root is None:
            self.root = Node(key)
            self._insert(self.root, key)

    def _insert(self, current, key):
        if key < current.val:
            if current.left is None:
                current.left = Node(key)
                self._insert(current.left, key)
            if current.right is None:
                current.right = Node(key)
                self._insert(current.right, key)

    def search(self, key):
        return self._search(self.root, key)

    def _search(self, current, key):
        if current is None or current.val == key:
            return current
        if key < current.val:
            return self._search(current.left, key)
        return self._search(current.right, key)

    def inorder_traversal(self, root):
        if root:
            print(root.val, end=' ')

# Example usage:
bst = BinarySearchTree()
bst.inorder_traversal(bst.root)  # Output: 3 5 7 10 15 20 25
print(  # Output: 
print(  # Output: None

This code defines a simple binary search tree with methods for inserting nodes, searching for nodes, and performing in-order traversal.

Types of Binary Search Trees

There are several variations of binary search trees, each with distinct characteristics:

  • Balanced Binary Search Tree: A binary search tree in which the heights of the left and right subtrees of any node differ by at most one. Examples include AVL trees and Red-Black trees.
  • AVL Tree: A self-balancing binary search tree where the difference in heights of left and right subtrees for any node is at most one. AVL trees maintain balance through rotations during insertion and deletion.
  • Red-Black Tree: A self-balancing binary search tree with an additional property that ensures the tree remains approximately balanced. Each node has a color attribute (either red or black) and the tree maintains balance through rotations and color changes during insertion and deletion.
  • Splay Tree: A self-adjusting binary search tree that moves frequently accessed elements closer to the root, improving access time for those elements.
  • Treap: A randomized binary search tree that maintains heap properties as well, balancing itself through random priorities assigned to each node.


Binary Search Trees are a fundamental data structure that provides efficient searching, insertion, and deletion operations. Understanding their components, properties, and variations is crucial for implementing various algorithms and solving complex problems.