Java代码实现构造一颗三叉链表表示的二叉树，实现采用先根、中根、后根、层次算法遍历二叉树，输出二叉树结点值

以下是Java代码实现构造一颗三叉链表表示的二叉树，实现采用先根、中根、后根、层次算法遍历二叉树，输出二叉树结点值的实现：

// 三叉链表表示的二叉树结点类
class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode parent;

    public TreeNode(int val) {
        this.val = val;
        this.left = null;
        this.right = null;
        this.parent = null;
    }
}

// 三叉链表表示的二叉树类
class BinaryTree {
    TreeNode root;

    // 构造函数
    public BinaryTree() {
        root = null;
    }

    // 插入结点
    public void insert(int val) {
        TreeNode newNode = new TreeNode(val);
        if (root == null) {
            root = newNode;
        } else {
            TreeNode current = root;
            while (true) {
                if (val < current.val) {
                    if (current.left == null) {
                        current.left = newNode;
                        newNode.parent = current;
                        break;
                    } else {
                        current = current.left;
                    }
                } else {
                    if (current.right == null) {
                        current.right = newNode;
                        newNode.parent = current;
                        break;
                    } else {
                        current = current.right;
                    }
                }
            }
        }
    }

    // 先根遍历
    public void preOrder(TreeNode node) {
        if (node != null) {
            System.out.print(node.val + " ");
            preOrder(node.left);
            preOrder(node.right);
        }
    }

    // 中根遍历
    public void inOrder(TreeNode node) {
        if (node != null) {
            inOrder(node.left);
            System.out.print(node.val + " ");
            inOrder(node.right);
        }
    }

    // 后根遍历
    public void postOrder(TreeNode node) {
        if (node != null) {
            postOrder(node.left);
            postOrder(node.right);
            System.out.print(node.val + " ");
        }
    }

    // 层次遍历
    public void levelOrder(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        if (root != null) {
            queue.offer(root);
        }
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            System.out.print(node.val + " ");
            if (node.left != null) {
                queue.offer(node.left);
            }
            if (node.right != null) {
                queue.offer(node.right);
            }
        }
    }

    // 测试函数
    public static void main(String[] args) {
        BinaryTree tree = new BinaryTree();
        tree.insert(6);
        tree.insert(3);
        tree.insert(8);
        tree.insert(2);
        tree.insert(5);
        tree.insert(7);
        tree.insert(9);

        System.out.print("先根遍历：");
        tree.preOrder(tree.root);
        System.out.println();

        System.out.print("中根遍历：");
        tree.inOrder(tree.root);
        System.out.println();

        System.out.print("后根遍历：");
        tree.postOrder(tree.root);
        System.out.println();

        System.out.print("层次遍历：");
        tree.levelOrder(tree.root);
        System.out.println();
    }
}

输出结果为：

先根遍历：6 3 2 5 8 7 9 
中根遍历：2 3 5 6 7 8 9 
后根遍历：2 5 3 7 9 8 6 
层次遍历：6 3 8 2 5 7 9