两种打印二叉树（队列）

使用标准输入输出

import java.util.*;
public class Main1 {
    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        String text = scanner.nextLine();
        scanner.close();

        Solution.TreeNode inputData = parseStringTreeNode(text);
        Solution solution = new Solution();
        Solution.TreeNode root = solution.invertTree(inputData);

        System.out.println(printTree(root));
    }

    private static String printTree(Solution.TreeNode root) {
        if (root == null) {
            return "[]";
        }

        StringBuilder sb = new StringBuilder();
        sb.append("[");
        Queue<Solution.TreeNode> queue = new LinkedList<>();
        queue.add(root);

        // 记录最后一个非 null 节点的位置
        int lastNonNullIndex = -1;
        List<String> nodeValues = new ArrayList<>();

        while (!queue.isEmpty()) {
            Solution.TreeNode current = queue.poll();
            if (current != null) {
                nodeValues.add(String.valueOf(current.val));
                lastNonNullIndex = nodeValues.size() - 1;
                queue.add(current.left);
                queue.add(current.right);
            } else {
                nodeValues.add("null");
            }
        }

        // 构建最终的字符串，只保留到最后一个非 null 节点为止
        for (int i = 0; i <= lastNonNullIndex; i++) {
            sb.append(nodeValues.get(i));
            if (i < lastNonNullIndex) {
                sb.append(", ");
            }
        }
        sb.append("]");
        return sb.toString();
    }

    private static Solution.TreeNode parseStringTreeNode(String literal) {
        if (literal == null || literal.length() <= 2) {
            return null;
        }

        // 去掉首尾的方括号，然后分割字符串得到字符串数组
        String[] elements = literal.substring(1, literal.length() - 1).split(",\\s*");

        // 处理特殊情况，如果数组为空
        if (elements.length == 0 || (elements.length == 1 && elements[0].isEmpty())) {
            return null;
        }

        // 创建根节点
        Solution.TreeNode root = new Solution.TreeNode(Integer.parseInt(elements[0]));
        Queue<Solution.TreeNode> queue = new LinkedList<>();
        queue.add(root);

        int i = 1;
        while (!queue.isEmpty() && i < elements.length) {
            Solution.TreeNode current = queue.poll();

            // 处理左子节点
            if (i < elements.length && !elements[i].equals("null")) {
                current.left = new Solution.TreeNode(Integer.parseInt(elements[i]));
                queue.add(current.left);
            }
            i++;

            // 处理右子节点
            if (i < elements.length && !elements[i].equals("null")) {
                current.right = new Solution.TreeNode(Integer.parseInt(elements[i]));
                queue.add(current.right);
            }
            i++;
        }

        return root;
    }
}

直接使用节点

import java.util.*;
public class Main1 {
    public static void main(String[] args) {

        Solution.TreeNode root = new Solution.TreeNode(1);
        root.left = new Solution.TreeNode(3);
        root.right = null;
        root.left.left = null;
        root.left.right = new Solution.TreeNode(2);

        System.out.println(printTree(root));
    }

private static String printTree(Solution.TreeNode root) {
    if (root == null) {
        return "[]";
    }

    StringBuilder sb = new StringBuilder();
    sb.append("[");
    Queue<Solution.TreeNode> queue = new LinkedList<>();
    queue.add(root);

    List<String> nodeValues = new ArrayList<>();

    while (!queue.isEmpty()) {
        Solution.TreeNode current = queue.poll();
        if (current != null) {
            nodeValues.add(String.valueOf(current.val));
            queue.add(current.right); // 注意先添加右子节点
            queue.add(current.left);  // 再添加左子节点
        } else {
            nodeValues.add("null");
        }
    }

    // 去除末尾的多余 "null"
    int lastNonNullIndex = nodeValues.size() - 1;
    while (lastNonNullIndex >= 0 && nodeValues.get(lastNonNullIndex).equals("null")) {
        lastNonNullIndex--;
    }

    // 构建最终的字符串，只保留到最后一个非 null 节点为止
    for (int i = 0; i <= lastNonNullIndex; i++) {
        sb.append(nodeValues.get(i));
        if (i < lastNonNullIndex) {
            sb.append(", ");
        }
    }
    sb.append("]");
    return sb.toString();
}

两个打印的方法有区别