What Is Preorder Traversal in Data Structure?


Heather Bennett

In data structure, preorder traversal is a method used to traverse and visit every node in a binary tree. It is a depth-first traversal technique where the root node is visited first, followed by the left subtree, and then the right subtree.

Understanding Preorder Traversal

Preorder traversal follows a specific order to visit each node in a binary tree. The process can be summarized as follows:

  • Visit the root node: In preorder traversal, we start by visiting the root node of the binary tree.
  • Traverse the left subtree: After visiting the root node, we move to the left subtree and perform a preorder traversal on it.
  • Traverse the right subtree: Once we finish traversing the entire left subtree, we move to the right subtree and perform a preorder traversal on it.

An Example of Preorder Traversal

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

       / \
      2   3
     / \   \
    4   5   6

If we perform a preorder traversal on this binary tree, the order in which nodes are visited would be:

  1. Visit node 1 (root)
  2. Visit node 2 (left child of root)
  3. Visit node 4 (left child of node 2)
  4. Visit node 5 (right child of node 2)
  5. Visit node 3 (right child of root)
  6. Visit node 6 (right child of node 3)

Implementing Preorder Traversal

To implement preorder traversal, we can use recursion or iteration. Here’s an example of how to implement it using recursion in Python:

def preorderTraversal(node):
    if node is None:
    # Visit the root node
    # Traverse the left subtree
    # Traverse the right subtree

This recursive implementation follows the same logic as described earlier. It first visits the root node, then recursively performs a preorder traversal on the left and right subtrees.


Preorder traversal is a fundamental method for visiting nodes in a binary tree. By understanding its order and implementing it correctly, you can effectively traverse and process every node in a tree data structure.

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