What Is Preorder in Data Structure?

In data structure, a preorder traversal is a popular method used to visit all the nodes of a binary tree. It is called “preorder” because it visits the nodes in a specific order: **parent node, left child node, right child node**.

## The Process of Preorder Traversal

When performing a preorder traversal, the algorithm follows these steps:

**Visit the current node:**Start with the root node and print its value.**Traverse the left subtree:**Move to the left child of the current node and repeat the process.**Traverse the right subtree:**Move to the right child of the current node and repeat the process.

To better understand how preorder traversal works, let’s consider an example binary tree:

A / \ B C / \ / \ D E F G

## An Example of Preorder Traversal

If we perform a preorder traversal on this binary tree, we would visit the nodes in this order: __A, B, D, E, C, F, G__. Let’s break it down step by step:

### Step 1: Visit Node A

We start at the root node A and print its value: A.

### Step 2: Traverse Left Subtree (Node B)

We move to Node B and visit it: B.

### Step 3: Traverse Left Subtree (Node D)

We move to Node D and visit it: D.

### Step 4: Backtrack to Parent (Node B) and Traverse Right Subtree (Node E)

Since Node D has no further children, we backtrack to its parent Node B. From there, we move to the right child Node E and visit it: E.

### Step 5: Backtrack to Parent (Node B) and Traverse Right Subtree (Node C)

Since Node E has no further children, we backtrack to its parent Node B. From there, we move to the right child Node C and visit it: C.

### Step 6: Traverse Left Subtree (Node F)

We move to Node F and visit it: F.

### Step 7: Backtrack to Parent (Node C) and Traverse Right Subtree (Node G)

Since Node F has no further children, we backtrack to its parent Node C. From there, we move to the right child Node G and visit it: G.

## Applications of Preorder Traversal

The preorder traversal algorithm is a fundamental technique in various data structure operations:

**Expression evaluation:**Preorder traversal can be used to evaluate prefix expressions where operators appear before their operands.**Creating a copy of a binary tree:**Preorder traversal can be used to create an identical copy of a given binary tree.**Building expression trees:**Preorder traversal can help construct expression trees from prefix expressions.

In conclusion, understanding preorder traversal is essential for anyone working with binary trees or related algorithms. By following the specific order of visiting nodes, you can efficiently process and manipulate tree structures in various applications.