What Is Pre-Order in Data Structure?


Angela Bailey

Data structures are an essential aspect of computer science and programming. They help organize and store data efficiently, allowing for easy retrieval and manipulation. One commonly used data structure is the pre-order traversal.

What is Pre-Order Traversal?

Pre-order traversal is a method of traversing or visiting each node in a binary tree. In this traversal technique, the root node is visited first, followed by the left subtree, and then the right subtree. It can be represented as follows:

    if root is null
    print root's data
    preOrder(root's left)
    preOrder(root's right)

Let’s break down the process step by step to understand it better:

Step 1:

The algorithm begins with checking if the current node is null or not. If it is null, that means we have reached the end of a branch or subtree, so we return.

Step 2:

Next, we print the data of the current node. This operation can vary depending on our specific requirements. We can output the data to the console, store it in an array or perform any other desired action.

Step 3:

Moving on to step three, we recursively call preOrder on the left subtree of the current node.

Step 4:

Finally, in step four, we recursively call preOrder on the right subtree of the current node.

By following these steps recursively for each node in a binary tree, we can traverse it in pre-order fashion.

Why Use Pre-Order Traversal?

Pre-order traversal has several applications in computer science and programming. One common use case is in parsing expressions or mathematical formulas. By traversing an expression tree in pre-order, we can extract the operands and operators in the correct order, allowing us to evaluate the expression correctly.

Another application of pre-order traversal is constructing a copy of a binary tree. By traversing the original tree in pre-order and creating new nodes with the same values, we can create an identical copy of the original tree.

Additionally, pre-order traversal can be used for creating prefix expressions. Prefix expressions are mathematical expressions where operators appear before their operands. Pre-order traversal allows us to obtain a prefix expression by visiting the root node first, followed by its left and right subtrees.


In conclusion, pre-order traversal is a fundamental technique used for visiting nodes in a binary tree. It follows a specific order: root, left subtree, right subtree. Understanding this traversal method is crucial for various applications such as parsing expressions, creating copies of trees, and generating prefix expressions.

By incorporating pre-order traversal into your programming toolkit, you can enhance your ability to manipulate data structures efficiently and effectively.

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