递归遍历 (前序、中序、后序遍历)

    /**
     * 前序遍历
     */
    public static void preOrder(TreeNode node) {
        if (Objects.isNull(node)) {
            return;
        }

        System.out.print(node.data + " ");
        preOrder(node.leftChild);
        preOrder(node.rightChild);
    }

    /**
     * 中序遍历
     */
    public static void inOrder(TreeNode node) {
        if (Objects.isNull(node)) {
            return;
        }

        inOrder(node.leftChild);
        System.out.print(node.data + " ");
        inOrder(node.rightChild);
    }

    /**
     * 后序遍历
     */
    public static void postOrder(TreeNode node) {
        if (Objects.isNull(node)) {
            return;
        }

        postOrder(node.leftChild);
        postOrder(node.rightChild);
        System.out.print(node.data + " ");
    }
    
    /**
     * 测试案例
     */
    public static void main(String[] args) {
        LinkedList<Integer> datas = new LinkedList<>(Arrays.asList(3,2,9,null,null,10,null,null,8,null,4));

        TreeNode treeNode = TreeNode.createTreeNode(datas);

        System.out.println("前序遍历");
        preOrder(treeNode);

        System.out.println();
        System.out.println("中序遍历");
        inOrder(treeNode);

        System.out.println();
        System.out.println("后序遍历");
        postOrder(treeNode);
    }

打印结果

前序遍历
3 2 9 10 8 4 

中序遍历
9 2 10 3 8 4 

后序遍历
9 10 2 4 8 3 

非递归遍历 (层序遍历)

    /**
     * 层序遍历
     */
    public static void stackOrder(TreeNode treeNode) {
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(treeNode);

        while (!queue.isEmpty()) {
            TreeNode poll = queue.poll();
            System.out.print(poll.data + " ");
            if (poll.leftChild != null) {
                queue.add(poll.leftChild);
            }
            if (poll.rightChild != null) {
                queue.add(poll.rightChild);
            }
        }
    }
    
    public static void main(String[] args){
        LinkedList<Integer> datas = new LinkedList<>(Arrays.asList(1,2,4,null,null,5,null,null,3,null,6));

        TreeNode treeNode = TreeNode.createTreeNode(datas);

        System.out.println("层序遍历: ");
        stackOrder(treeNode);
    }

打印结果

层序遍历: 
1 2 3 4 5 6