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# What Is the Time Complexity of Tree Traversal?

Last modified: August 25, 2021

## 1. Overview

In Computer Science, linear data structures can be traversed in only one logical way. However, a tree data structure can be traversed in several different ways.

**In this tutorial, we’ll discuss various ways to traverse a tree and the time complexity of such an operation.**

## 2. What Is Tree Traversal?

**Tree traversal is the process of visiting each node in a tree exactly once.** Now from a current node, there might be more than one node we can reach. Therefore, it’s possible to have several different orders in which the nodes are visited. Let’s take an example:

The nodes of this tree can be traversed in many different ways. Some traversal orders are:

- Order 1:
- Order 2:
- Order 3:

**Hence depending on the nodes being visited, the traversal is classified into two categories: depth-first traversal and level order or breadth-first traversal.**

The depth-first traversal can be further divided into three types of traversal: in-order, pre-order, and post-order traversal.

## 3. Tree Traversals Techniques

According to in-order traversal, **we visit the nodes in the left subtree first, followed by the root node and the nodes in the right subtree.** We first search for the left leaf node of the left subtree to start the in-order traversal.

In pre-order traversal, **we visit the root node first, then the left subtree, and finally the right subtree.**

In the case of post-order traversal, **we visit the left subtree, then the right subtree, and finally the root node.**

Finally, for level-order or breadth-first traversal, **we visit all the nodes in a tree level by level fashion.** We start with the root node and goes to the next level to visit the nodes.

## 4. Example

Now we’re taking an example tree, and let’s find the various traversals of the tree:

If we apply in-order traversal in this example tree,** the in-order traversal of this tree would be**

In the case of pre-order traversal, we visit the root node first.** Hence the pre-order traversal of this tree would be:**

In the post-order traversal, we traverse the root node at the end. **Therefore the post-order traversal of this tree would be:**

**The level-order traversal of the example tree would be:**

## 5. Time Complexity of the Tree Traversals

In the general case, all the traversal algorithms visit each node in a tree exactly once. Therefore the time complexity of all the traversal algorithms would be when a tree contains nodes.

In order to verify the overall time complexity, we’re taking a corner case, and we’re going to find the time complexity to visit all the nodes. If a tree has nodes, then the time complexity of the tree can be defined as:

is the number of nodes on the left side of the tree, and denotes a constant time. **Now let’s assume that the given tree is a right-skewed tree.** In the case of a right-skewed tree, the left of the tree will be empty. So :

If we continue, we’ll get:

denotes traversing an empty tree which takes constant time:

**Therefore, we verified that all the tree traversal algorithms take** **time.**

## 6. Conclusion

In this tutorial, we learned several ways to traverse a tree. We presented algorithms for each traversal and demonstrated with an example. Finally, we analyzed the time complexity of the traversal algorithms.