Binary tree InOrder traversal in Java using Recursion (With Example)

The InOrder traversal is one of the three popular ways to traverse a binary tree data structure, the other two being the preOrder and postOrder. During the in-order traversal algorithm, the left subtree is explored first, followed by root, and finally nodes on the right subtree. You start traversal from root then go to the left node, then again go to the left node until you reach a leaf node. At that point in time, you print the value of the node or mark it visited and moves to the right subtree. Continuing the same algorithm until all nodes of the binary tree are visited. The InOrder traversal is also known as the left-node-right or left-root-right traversal or LNR traversal algorithm. Similar to the preOrder algorithm, it is also a depth-first algorithm because it explores the depth of a binary tree before exploring siblings. Since it is one of the fundamental binary tree algorithms it's quite popular in programming interviews. These traversal algorithms are also the basis to learn more advanced binary tree algorithms, hence every programmer should learn, understand, and know how to implement in-order and other traversal algorithms. The easiest way to implement the inOrder traversal algorithm in Java or any programming language is by using recursion. Since the binary tree is a recursive data structure, recursion is the natural choice for solving a tree-based problem. The inOrder() method in the BinaryTree class implements the logic to traverse a binary tree using recursion. From the Interview point of view, InOrder traversal is extremely important because it also prints nodes of a binary search tree in the sorted order but only if the given tree is a binary search tree. If you remember, in BST, the value of nodes in the left subtree is lower than the root, and the values of nodes on the right subtree are higher than the root. The In order traversal literally means IN order i.e notes are printed in the order or sorted order. Btw, even though these three algorithms (pre-order, in-order, and post-order) are popular binary tree traversal algorithms but they are not the only ones. You also have other breadth-first ways to traverse a binary tree e.g. level order traversal

The Recursive algorithm to implement InOrder traversal of a Binary tree

The recursive algorithm of inorder traversal is very simple. You just need to call the inOrder() method of BinaryTree class in the order you want to visit the tree. What is most important is to include the base case, which is key to any recursive algorithm. For example, in this problem, the base case is you reach the leaf node and there is no more node to explore, at that point of time recursion starts to wind down. Here are the exact steps to traverse the binary tree using InOrder traversal:

  1. visit left node

  2. print value of the root

  3. visit right node

and here is the sample code to implement this algorithm using recursion in Java:

private void inOrder(TreeNode node) {     
    if (node == null) 
        {       
        return;     
        }      
    inOrder(node.left);     
    System.out.printf("%s ", node.data);     
    inOrder(node.right); 
 }

Similar to the preOrder() method in the last example, there is another inOrder() method which exposes inorder traversal to the public and calls this private method which actually performs the InOrder traversal. This is the standard way to write a recursive method that takes input, it makes it easier for a client to call the method.

public void inOrder() {     
    inOrder(root); 
 }

You can see that we start with root and then recursive call the inOrder() method with node.left, which means we are going down on the left subtree until we hit node == null, which means the last node was a leaf node. At this point in time, the inOrder() method will return and execute the next line, which prints the node.data. After that it's again recursive inOrder() call with node.right, which will initiate the same process again.

Java Program to implement InOrder traversal of a Binary tree

Here is our complete solution to the inorder traversal algorithm in Java. This program uses a recursive algorithm to print the value of all nodes of a binary tree using InOrder traversal. As I have told you before, during the in-order traversal value of the left subtree is printed first, followed by root and right subtree. If you are interested in the iterative algorithm, you can further check this