题解 | #牛群分层排列# Java

层序遍历+每层得到一个StringBuilder对象存入结果数组

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param root TreeNode类
     * @return string字符串一维数组
     */
    public String[] levelOrder (TreeNode root) {
        if (root == null) {
            return new String[0];
        }

        List<String> result = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();

        queue.offer(root); // 将根节点入队
        StringBuilder sb;
        while (!queue.isEmpty()) {
            int levelSize = queue.size();
            sb = new StringBuilder();

            // 遍历当前层的所有节点
            for (int i = 0; i < levelSize; i++) {
                TreeNode node = queue.poll();
                sb.append(node.val);

                // 将当前节点的左右子节点入队
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
            result.add(sb.toString());
        }

        // 将结果转换为字符串数组
        String[] levelOrder = new String[result.size()];
        for (int i = 0; i < result.size(); i++) {
            levelOrder[i] = result.get(i);
        }

        return levelOrder;
    }
}