Java: Get the preorder traversal of its nodes' values of a given a binary tree.

Last update on May 12 2025 13:12:31 (UTC/GMT +8 hours)

Preorder Binary Tree Traversal

Write a Java program to get the preorder traversal of the values of the nodes in a binary tree.
Example:
Expected output: 10 20 40 50 30

Sample Binary Tree

Preorder Traversal:

Sample Solution:

Java Code:

class Node
{
    int key;
    Node left, right;

    public Node(int item)
    {
        // Constructor to create a new Node with the given item
        key = item;
        left = right = null;
    }
}

class BinaryTree
{
    Node root;

    BinaryTree()
    {
        // Constructor to create an empty binary tree
        root = null;
    }

    void print_Preorder(Node node)
    {
        if (node == null)
            return;

        // Print the key of the current node
        System.out.print(node.key + " ");

        // Recursively print the left subtree in preorder
        print_Preorder(node.left);

        // Recursively print the right subtree in preorder
        print_Preorder(node.right);
    }

    void print_Preorder()  
    {   
        // Wrapper method to start printing the tree in preorder
        print_Preorder(root); 
    }

    public static void main(String[] args)
    {
        BinaryTree tree = new BinaryTree();
        
        // Create a binary tree with nodes and keys
        tree.root = new Node(55);
        tree.root.left = new Node(21);
        tree.root.right = new Node(80);
        tree.root.left.left = new Node(9);
        tree.root.left.right = new Node(29);
        tree.root.right.left = new Node(76);
        tree.root.right.right = new Node(91);

        // Display a message and initiate the preorder traversal
        System.out.println("Preorder traversal of binary tree is: ");
        tree.print_Preorder();
    }
}

Sample Output:

Preorder traversal of binary tree is: 
55 21 9 29 80 76 91  

Flowchart:

For more Practice: Solve these Related Problems:

• Modify the program to return a list instead of printing values.
• Write a program to perform a non-recursive preorder traversal.
• Modify the program to perform a Morris traversal.
• Write a program to print preorder traversal using level-order approach.

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