Approach
We will follow the in-order traversal recursively on the binary tree for double-order traversal.
Steps to be followed to print the Double Order Traversal:
Start the Inorder traversal from the root.
Print the current node’s value.
Recursively call for the left subtree.
Print the current node again.
Recursively call for the right subtree.
After performing the above steps, the Double order Traversal of the Binary Tree will get printed.
Implementation
class Solution {
// Node
static class node
{
char data;
node left, right;
};
// Function to create new node
static node newNode(char c)
{
node n = new node();
n.data = c;
n.left = null;
n.right = null;
return n;
}
// Function to print Double Order traversal
static void doubleOrder(node root)
{
if (root == null)
return;
// Print Node Value
System.out.print(root.data + " ");
// Traverse Left Subtree
doubleOrder(root.left);
// Print Node Value
System.out.print(root.data + " ");
// Traverse Right SubTree
doubleOrder(root.right);
}
// Main function
public static void main(String[] args)
{
node root = newNode('1');
root.left = newNode('2');
root.right = newNode('3');
root.left.left = newNode('4');
root.left.right = newNode('5');
root.right.right = newNode('6');
doubleOrder(root);
}
}

You can also try this code with Online Java Compiler
Run Code
Output
1 2 4 4 2 5 5 1 3 3 6 6
Time Complexity: The time complexity of the above approach is O(N) because we are doing the in-order traversal of the Tree.
Space Complexity: The space complexity of the above approach is O(1) because we are not using any auxiliary space.
Check out this problem - Mirror A Binary Tree
Frequently Asked Questions
What do you mean by a binary tree?
It is a non-linear data structure with nodes as elements connected in a tree-like structure where each node should have two children.
How to traverse a binary tree?
The tree could be traversed using Inorder, Preorder, and Postorder tree traversals.
Conclusion
In this article, we have covered the following things:
- What precisely a binary tree is.
- Meaning of the double order traversal in a binary tree.
- Implementation of double-order binary tree traversal.
Don't stop here; prepare the topic trees in-depth for product-based companies' technical rounds.
Also, don’t forget to practice the wide variety of DSA problems frequently asked in interview rounds readily available on Coding Ninjas Studio.
Recommended Reading:
Recommended problems -
Do check out The Interview guide for Product Based Companies as well as some of the Popular Interview Problems from Top companies like Amazon, Adobe, Google, Uber, Microsoft, etc. on Coding Ninjas Studio.
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Happy coding!!