题解 | #牛群分层排列#

简单的层序遍历：

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 {
    public String[] levelOrder (TreeNode root) {
        if(root == null) return new String[0];
        List<String> res = new ArrayList<>();
        Deque<TreeNode> deque = new LinkedList<>();
        deque.offer(root);
        res.add(String.valueOf(root.val));
        while(!deque.isEmpty()){
            int size = deque.size();
            StringBuilder sb = new StringBuilder();
            for(int i = 0;i<size;i++){
                TreeNode node = deque.poll();
                if(node.left != null){
                    sb.append(node.left.val);
                    deque.offer(node.left);
                }
                if(node.right != null){
                    sb.append(node.right.val);
                    deque.offer(node.right);
                }
            }
		  // 注意此处的条件，数组中不能存在空字符串
            if(sb.length() >= 1) res.add(sb.toString());
        }
        return res.toArray(new String[res.size()]);
    }
}