What Is Expression Tree in Data Structure?


Angela Bailey

An expression tree is a specific type of binary tree used to represent mathematical expressions or logical expressions. It provides a way to store expressions in a tree-like structure, where each node represents an operator or an operand. In this article, we will delve into the concept of expression trees and understand their significance in data structures.

Components of an Expression Tree

An expression tree consists of nodes that represent operands and operators. The operands are leaf nodes, while the operators are internal nodes. Each internal node has exactly two children, which can be either other operators or operands.

To illustrate this better, let’s consider the following arithmetic expression:

8 + 3 * 5 – 2

In the corresponding expression tree, the numbers 8, 3, 5, and 2 would be represented as leaf nodes. The addition (+), multiplication (*), and subtraction (-) operators would be represented as internal nodes.

Advantages of Expression Trees

Expression trees offer several advantages when it comes to evaluating mathematical expressions:

  • Efficient Evaluation: Expression trees provide an efficient way to evaluate complex expressions by simplifying them into smaller sub-expressions.
  • Infix to Postfix Conversion: Expression trees can be used to convert infix expressions (where operators are placed between operands) to postfix expressions (where operators are placed after operands), making evaluation easier.
  • Parsing Expressions: Expression trees can be used for parsing mathematical expressions and extracting meaningful information from them.

Creating an Expression Tree

To create an expression tree from a given expression, we typically use a recursive algorithm. Here’s a simplified algorithm to build an expression tree:

  1. If the given expression is empty, return null.
  2. Scan the expression from right to left and find the rightmost operator. This operator will become the root of the expression tree.
  3. The operands on the left side of the operator will be recursively set as the left child of the root.
  4. The operands on the right side of the operator will be recursively set as the right child of the root.
  5. Return the root node.

Using Expression Trees for Evaluation

Once we have built an expression tree, we can evaluate it by performing a postorder traversal. In postorder traversal, we first visit the left subtree, then visit the right subtree, and finally process the root node.

Here’s a simplified algorithm for evaluating an expression tree:

  1. If the given node is null, return 0 (or any appropriate value).
  2. If it is a leaf node, return its value.
  3. Recursively evaluate its left subtree and store the result in variable ‘left’.
  4. Recursively evaluate its right subtree and store the result in variable ‘right’.
  5. Perform appropriate arithmetic operation based on its operator and return the result.


In summary, an expression tree is a useful data structure for representing mathematical expressions in a more organized manner. It allows efficient evaluation of complex expressions and assists in various operations like infix to postfix conversion and parsing expressions. By understanding how to create and evaluate expression trees, you can enhance your understanding of data structures and algorithms related to mathematical computations.

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