做题博客链接

https://blog.csdn.net/qq_43349112/article/details/108542248

题目链接

https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-iii-lcof/

描述

请实现一个函数按照之字形顺序打印二叉树，即第一行按照从左到右的顺序打印，第二层按照从右到左的顺序打印，第
三行再按照从左到右的顺序打印，其他行以此类推。


提示：

节点总数 <= 1000

示例

例如:

给定二叉树: [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7
返回其层次遍历结果：

[
  [3],
  [20,9],
  [15,7]
]

初始代码模板

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    
    
    public List<List<Integer>> levelOrder(TreeNode root) {
    
    
      
    }
}

代码

推荐题解：
https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-iii-lcof/solution/mian-shi-ti-32-iii-cong-shang-dao-xia-da-yin-er–3/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    
    
    public List<List<Integer>> levelOrder(TreeNode root) {
    
    
        List<List<Integer>> res = new LinkedList<>();
        Deque<TreeNode> queue = new ArrayDeque<>();
        if (root != null) {
    
    
            queue.offer(root);
        }

        while (!queue.isEmpty()) {
    
    
            LinkedList<Integer> tmp = new LinkedList<>();
            for (int size = queue.size(); size > 0; size--) {
    
    
                TreeNode node = queue.poll();
                if((res.size() & 1) == 0) tmp.addLast(node.val);
                else tmp.addFirst(node.val);
                if(node.left != null) queue.add(node.left);
                if(node.right != null) queue.add(node.right);
            }
            res.add(tmp);
        }

        return res;
    }
}