参考: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 */ } }