Inorder traversal is a crucial algorithm for working with binary trees, particularly binary search trees (BSTs). It‘s a fundamental concept in computer science that every aspiring software engineer should aim to understand thoroughly. In this comprehensive guide, we‘ll explore inorder traversal in depth, with plenty of examples, code implementations, and analysis.
As a digital technology expert, I‘ve seen firsthand how a solid grasp of tree traversal algorithms like inorder traversal can distinguish candidates during technical interviews and on the job. Whether you‘re a computer science student, a coding bootcamp graduate, or a selftaught programmer, mastering inorder traversal will serve you well in your journey as a software engineer.
Binary Search Trees: A Quick Refresher
Before diving into inorder traversal, let‘s review some key properties of binary search trees. A BST is a binary tree where, for each node:
 The left subtree only contains nodes with keys less than the node‘s key
 The right subtree only contains nodes with keys greater than the node‘s key
 Both the left and right subtrees must also be binary search trees
Thanks to these constraints, BSTs provide an efficient way to store and retrieve sorted data. Common operations like search, insertion, and deletion can be performed in O(log n) average time complexity, where n is the number of nodes in the tree.
Here‘s an example of a valid BST:
8
/ \
3 10
/ \ \
1 6 14
/ \ /
4 7 13
Notice how, for each node, all keys in its left subtree are smaller and all keys in its right subtree are larger. This property is what makes inorder traversal so useful for BSTs.
Inorder Traversal: The Basics
An inorder traversal of a binary tree visits each node in the following order:
 Recursively traverse the left subtree
 Visit the current node
 Recursively traverse the right subtree
When performed on a BST, an inorder traversal visits the nodes in ascending order of their keys. This is because we completely exhaust the left subtree (which contains smaller keys) before visiting the node itself, and then move on to the right subtree (which contains larger keys).
Here‘s the inorder traversal sequence for the example BST from the previous section:
1, 3, 4, 6, 7, 8, 10, 13, 14
As expected, this is the sorted order of the keys in the tree.
Recursive Implementation
The most straightforward way to implement inorder traversal is using recursion. Here‘s how it looks in Python:
def inorder_traversal(root):
if root is None:
return
inorder_traversal(root.left)
print(root.val)
inorder_traversal(root.right)
And here‘s the equivalent code in Java:
void inorderTraversal(TreeNode root) {
if (root == null) {
return;
}
inorderTraversal(root.left);
System.out.println(root.val);
inorderTraversal(root.right);
}
The recursive implementation closely mirrors the definition of inorder traversal. We first recursively traverse the left subtree, then visit the current node (in this case, printing its value), and finally recursively traverse the right subtree.
The base case for the recursion is when root
is None
(or null
in Java), meaning we‘ve exhausted a branch of the tree. At that point, we simply return without doing anything.
Time and Space Complexity Analysis
Let‘s analyze the time and space complexity of the recursive inorder traversal algorithm.
For time complexity, note that we visit each node exactly once. If the tree has n nodes, the time complexity is O(n), regardless of the tree‘s structure.
The space complexity is determined by the maximum depth of the recursive call stack. In the worst case, the tree is a linked list (i.e., each node has only one child), and the recursive stack will contain all n nodes, yielding O(n) space complexity.
However, for a balanced BST, the recursive stack will only contain O(log n) nodes at any given time, corresponding to the height of the tree. Therefore, the space complexity is O(log n) for balanced BSTs and O(n) in the worst case.
It‘s worth noting that the recursive implementation is limited by the maximum recursion depth allowed by the language or system. For extremely deep trees, a recursive solution might lead to a stack overflow error. In such cases, an iterative approach might be preferable.
Iterative Implementation
We can also implement inorder traversal iteratively using an explicit stack data structure. The idea is to simulate the recursive process by maintaining our own stack of nodes.
Here‘s the iterative version in Python:
def inorder_traversal(root):
stack = []
curr = root
while curr is not None or len(stack) > 0:
while curr is not None:
stack.append(curr)
curr = curr.left
curr = stack.pop()
print(curr.val)
curr = curr.right
And in Java:
void inorderTraversal(TreeNode root) {
Stack<TreeNode> stack = new Stack<>();
TreeNode curr = root;
while (curr != null  !stack.isEmpty()) {
while (curr != null) {
stack.push(curr);
curr = curr.left;
}
curr = stack.pop();
System.out.println(curr.val);
curr = curr.right;
}
}
The iterative approach maintains a stack of nodes and a pointer curr
to the current node. We start by pushing the root onto the stack and moving curr
to the leftmost node. We keep going left as far as possible, pushing each node onto the stack.
When we can‘t go left anymore, we pop a node off the stack, print its value, and then move curr
to its right child. We repeat this process until the stack is empty and curr
is null
, indicating that we‘ve traversed the entire tree.
The time complexity of the iterative version is still O(n) since we visit each node once. The space complexity is also O(h) where h is the height of the tree, as the stack will contain at most h nodes at any given time. For a balanced BST, this means O(log n) space complexity.
Applications of Inorder Traversal
Inorder traversal has numerous applications, especially when working with BSTs. Let‘s explore a few common use cases.
Outputting Sorted Data
Since inorder traversal visits the nodes of a BST in ascending order, it‘s an efficient way to retrieve the tree‘s elements in sorted order. This can be useful when you need to process or display the data in a sorted manner.
For example, consider a BST that stores the scores of players in a game. An inorder traversal of this tree would give us the scores in ascending order, which we could then use to display a leaderboard.
Constructing a BST from a Sorted Array
Given a sorted array, we can construct a balanced BST by recursively selecting the middle element as the root and then constructing the left and right subtrees from the subarrays to the left and right of the middle element.
Inorder traversal can be used to verify the correctness of the constructed BST. If the inorder traversal of the constructed tree matches the original sorted array, we can be confident that the tree was built correctly.
Here‘s the Python code to construct a balanced BST from a sorted array:
def sorted_array_to_bst(arr):
if not arr:
return None
mid = len(arr) // 2
root = TreeNode(arr[mid])
root.left = sorted_array_to_bst(arr[:mid])
root.right = sorted_array_to_bst(arr[mid+1:])
return root
And here‘s how we could verify the result using inorder traversal:
def is_valid_bst(root, arr):
inorder = []
inorder_traversal(root, inorder)
return inorder == arr
def inorder_traversal(root, result):
if root is None:
return
inorder_traversal(root.left, result)
result.append(root.val)
inorder_traversal(root.right, result)
Validating a BST
A binary tree is a valid BST if, for each node, all nodes in its left subtree have keys less than its key, and all nodes in its right subtree have keys greater than its key. We can check this property using inorder traversal.
The idea is to perform an inorder traversal and keep track of the previously visited node. If we ever encounter a node whose key is less than or equal to the key of the previous node, the tree is not a valid BST.
Here‘s the Python code:
def is_valid_bst(root):
prev = None
def inorder(node):
nonlocal prev
if node is None:
return True
if not inorder(node.left):
return False
if prev is not None and node.val <= prev.val:
return False
prev = node
return inorder(node.right)
return inorder(root)
And here‘s the Java version:
class Solution {
TreeNode prev = null;
public boolean isValidBST(TreeNode root) {
return inorder(root);
}
private boolean inorder(TreeNode node) {
if (node == null) {
return true;
}
if (!inorder(node.left)) {
return false;
}
if (prev != null && node.val <= prev.val) {
return false;
}
prev = node;
return inorder(node.right);
}
}
Converting Between BSTs and Arrays/Linked Lists
Since an inorder traversal of a BST visits the nodes in sorted order, we can easily convert a BST to a sorted array or linked list by performing an inorder traversal and adding each node to the result data structure.
Conversely, we can convert a sorted array or linked list to a balanced BST using the sorted_array_to_bst
function described earlier.
These conversions can be useful when we need to perform operations that are more efficient on one data structure than the other. For example, we might convert a BST to an array to efficiently access elements by index, or convert a sorted array to a BST to efficiently search for a specific value.
Threaded Binary Trees and Morris Traversal
A threaded binary tree is a variant of a binary tree where we use the null pointers of leaf nodes to store "threads" that point to the inorder successor (or predecessor) of the node. This allows us to perform an inorder traversal without using a stack or recursion.
The Morris traversal algorithm leverages threaded binary trees to achieve O(n) time complexity and O(1) space complexity for inorder traversal. The idea is to modify the tree during the traversal to create temporary links between nodes, allowing us to backtrack without a stack.
Here‘s the Python implementation of Morris traversal:
def morris_inorder_traversal(root):
curr = root
while curr is not None:
if curr.left is None:
print(curr.val)
curr = curr.right
else:
pre = curr.left
while pre.right is not None and pre.right is not curr:
pre = pre.right
if pre.right is None:
pre.right = curr
curr = curr.left
else:
pre.right = None
print(curr.val)
curr = curr.right
While the Morris traversal algorithm is more complex than the basic recursive and iterative approaches, it can be useful in situations where space efficiency is paramount, such as in embedded systems or when working with extremely large trees.
RealWorld Examples
Inorder traversal is a fundamental operation in many realworld software systems. Here are a few examples:

Expression Trees: Inorder traversal is used to evaluate arithmetic expressions represented as binary trees. In an expression tree, leaf nodes represent operands, and internal nodes represent operators. An inorder traversal of an expression tree yields the expression in infix notation.

File System Traversal: File systems are often represented as tree structures, with directories as internal nodes and files as leaf nodes. An inorder traversal of a file system tree can be used to list all files in alphabetical order.

Database Indexing: Many database systems use BSTs or balanced variants like AVL trees and RedBlack trees to index data for efficient search and retrieval. Inorder traversal is used to output the indexed data in sorted order.
Conclusion
Inorder traversal is a vital tool in the toolkit of any computer scientist or software engineer. Its simplicity and efficiency make it a goto algorithm for a wide range of problems involving binary trees and BSTs.
Throughout this guide, we‘ve covered the fundamentals of inorder traversal, including its definition, recursive and iterative implementations, time and space complexity analysis, and common applications. We‘ve also explored some more advanced topics like threaded binary trees and Morris traversal.
Mastering inorder traversal is not just about memorizing code, but about understanding the underlying principles and recognizing when and how to apply them. It‘s a skill that will serve you well in technical interviews, in your projects, and throughout your career as a software engineer.
As with any algorithm, the best way to truly understand inorder traversal is to practice implementing it yourself. Try working through the examples in this guide, and then tackle some problems on coding platforms like LeetCode or HackerRank. With time and practice, inorder traversal will become second nature.
Happy coding!
References

"Binary Search Tree." Wikipedia, Wikimedia Foundation, 6 June 2023, en.wikipedia.org/wiki/Binary_search_tree.

"Tree Traversal." Wikipedia, Wikimedia Foundation, 31 May 2023, en.wikipedia.org/wiki/Tree_traversal.

"Morris InOrder Traversal Using Threaded Binary Tree." Geeks for Geeks, 11 Dec. 2022, www.geeksforgeeks.org/morrisinordertraversalusingthreadedbinarytree/.

Chakraborty, Debasish. "Expression Tree." Geeks for Geeks, 16 Sept. 2021, www.geeksforgeeks.org/expressiontree/.

Harikrishnan, N.K. "File System Traversal." Geeks for Geeks, 28 Jan. 2023, www.geeksforgeeks.org/filesystemtraversal/.

"Database Index." Wikipedia, Wikimedia Foundation, 8 June 2023, en.wikipedia.org/wiki/Database_index.