參考：https://www.geeksforgeeks.org/binary-tree-set-1-introduction/

/* Class containing left and right child of current
node and key value*/
class Node
{
    int key;
    Node left, right;

    public Node(int item)
    {
        key = item;
        left = right = null;
    }
}

// A Java program to introduce Binary Tree
 

class BinaryTree
{
    // Root of Binary Tree
    Node root;

    // Constructors
    BinaryTree(int key)
    {
        root = new Node(key);
    }

    BinaryTree()
    {
        root = null;
    }

    public static void main(String[] args)
    {
        BinaryTree tree = new BinaryTree();

        /*
 
create root*/
        tree.root = new Node(1);

        /* following is the tree after above statement

            1
            / \
        null null     */

        tree.root.left = new Node(2);
        tree.root.right = new Node(3);

        /* 2 and 3 become left and right children of 1
            1
            /     \
        2     3
        / \     / \
    null null null null 
 
*/


        tree.root.left.left = new Node(4);
        /* 4 becomes left child of 2
                    1
                /     \
            2         3
            / \     / \
            4 null null null
        / \
        null null
        */
    }
}