如何打印二叉树

Question

I've been trying to switch over to Java from Node and one thing I'm wondering about is how to print an object such as a binary Tree in a similar format to how node would display it. For instance, my binary tree initiation code is as follows:

public class BinaryTree {
    int data;
    BinaryTree left, right;

    public static void main(String[] args) {
        BinaryTree tree = new BinaryTree(1);
        tree= new BinaryTree(1);
        tree.left = new BinaryTree(2);
        tree.right= new BinaryTree(3);
        tree.left.right = new BinaryTree(4);
        System.out.println(tree); // output -> BinaryTree@4554617c
    }

    public BinaryTree(int data) {
      super();
      int val;
      this.left = this.right = null;
  }
}

In node, this binary tree would be displayed as the following:

TreeNode {
  val: 1,
  right: TreeNode { val: 3, right: null, left: null },
  left:
   TreeNode {
     val: 2,
     right: TreeNode { val: 4, right: null, left: null },
     left: null } }

However in Java, when I do System.out.println(tree);

the output -> BinaryTree@4554617c

What is the proper way to print my BinaryTree and what's a good way to do this? Is there a way to print the tree in a JSON format?

Answer 1

Printing tree will give you the memory address of the main tree node. If you want to print the content of the tree, you will need to implement a recursive print method and recurse over each node in the tree.
If the node is a final node (no right or left tree) print the content of that node. Otherwise move down the tree. You can print on the way down the tree or on the way back depending on how you want the tree to look.
I hope I understood the question correctly.

Answer 2

I found this solution on stack overflow and modified it...

public static class TreeNode
{
    int value;
    TreeNode leftChildren;
    TreeNode rightChildren;

    public TreeNode(int value, TreeNode left, TreeNode right)
    {
        this.value = value;
        this.leftChildren = left;
        this.rightChildren = right;
    }

    public addLeftNode(TreeNode node)
    {
        this.leftChildren = node;
    }

    public addRightNode(TreeNode node)
    {
        this.rightChildren = node;
    }

    public void print(String prefix)
    {
        print(prefix, true);
    }

    private void print(String prefix, boolean isTail)
    {
        System.out.println(prefix + (isTail ? "└── " : "├── ") + this.toString());

        if(null != leftChildren)
        {
            leftChildren.print(prefix + (isTail ? "    " : "│   "), (null == rightChildren ? true : false));
        }

        if(null != rightChildren)
        {
            rightChildren.print(prefix + (isTail ?"    " : "│   "), true);
        }

    }

    @Override public String toString()
    {
        return "TreeNode { Value: " + this.value + "}";
    }
}

The first node represents the left and the second node the right node in th binary tree.

EDIT: You can use this like this:

TreeNode root = new TreeNode(22, null, null);

root.addLeftNode(new TreeNode(15, null, null));
root.addRightNode(new TreeNode(35, new TreeNode(27, null, null), null));

root.print(); // This will print recursively all sub-nodes

Alternate you can write a wrapper class like this:

public class BinaryTree
{
    private TreeNode root;

    // implementation of getters and setters

    public void print()
    {
        this.root.print();
    }
}