Pre-Order Traversal in Binary Tree not able to understand

Question

So, I'm trying to create binary tree using static data.

like below -

        (1)
        / \
       /   \
     (2)   (3)
     / \
    /   \
  (4)   (5)
          \
           \
           (6)

Here is the TreeNode.java

package sandeep.topics.binaryTree;

public class TreeNode {
    int value;
    TreeNode left, right;
    
    public TreeNode(int value) {
        this.value = value;
        left = null;
        right = null;
    }
}

& the BinaryTree.java

package sandeep.topics.binaryTrees;

public class BinaryTree {
    static TreeNode root;
    public BinaryTree() {
        
    }
    public BinaryTree(int value) {
        root = new TreeNode(value);
    }
    public static void main(String[] args) {
        BinaryTree binaryTree = new BinaryTree();
        // level 0
        binaryTree.root = new TreeNode(1);
        
        // level 1
        TreeNode level_1_left = binaryTree.createLeftChild(root, 2);
        TreeNode level_1_right = binaryTree.createRightChild(root, 3);
        
        // level 2
        TreeNode level_2_left = binaryTree.createLeftChild(level_1_left, 4);
        TreeNode level_2_right = binaryTree.createRightChild(level_1_right, 5);
        
        // level 3
        TreeNode level_3_right =  binaryTree.createRightChild(level_2_right, 6);
        
        
        printTree(root, "root", 1);
    }
    private static TreeNode createLeftChild(TreeNode root, int value) {
        root.left = new TreeNode(value);
        return root.left;
    }
    
    private static TreeNode createRightChild(TreeNode root, int value) {
        root.right = new TreeNode(value);
        return root.right;
    }
    private static void printTree(TreeNode root, String type, int level) {
        if(root == null) {
            return;
        }
        System.out.println(root.value + " : " + type + " : " + level);
        printTree(root.left, "left", level+1);
        printTree(root.right, "right", level+1);
    }
}

Very simple code that can create root, children as per level and print them.

Output shown as -

(Columns; 1st- node value, 2nd - node type & 3rd - level)


1 : root : 1
2 : left : 2
4 : left : 3
3 : right : 2
5 : right : 3
6 : right : 4

Shows - 1 2 4 3 5 6.

But I think, the correct pre-order traversal should be 1 2 4 5 6 3.

Not sure, what am i doing wrong though!

Answer 1

Your declaration of tree does not match your picture, I guess you wanted to type

TreeNode level_2_right = binaryTree.createRightChild(level_1_left, 5);

Otherwise the program seems correct (except that small improvements are possible - unnecessary constructor or assignments etc... it's usually good practice to declare TreeNode as immutable with children passed in constructor, which better visualizes tree structure using new TreeNode(1,new TreeNode(2,new TreeNode(4)...),new TreeNode(3)), ideally with proper indentation).