leetcode 剑指Offer 32-I.从上到下打印二叉树 Java

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-lcof/

描述

从上到下打印出二叉树的每个节点，同一层的节点按照从左到右的顺序打印。


提示：

节点总数 <= 1000

示例

给定二叉树: [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

返回：

[3,9,20,15,7]

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 int[] levelOrder(TreeNode root) {
    
    
       
    }
}

代码

推荐题解：
https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-lcof/solution/mian-shi-ti-32-i-cong-shang-dao-xia-da-yin-er-ch-4/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    
    
    public int[] levelOrder(TreeNode root) {
    
    
        if (root == null) {
    
    
            return new int[0];
        }

        ArrayList<Integer> list = new ArrayList<>();
        Queue<TreeNode> queue = new ArrayDeque<>();
        queue.offer(root);

        while (!queue.isEmpty()) {
    
    
            for (int size = queue.size(); size > 0; size--) {
    
    
                TreeNode node = queue.poll();
                list.add(node.val);

                if (node.left != null) {
    
    
                    queue.offer(node.left);
                }
                if (node.right != null) {
    
    
                    queue.offer(node.right);
                }
            }
        }

        int[] res = new int[list.size()];
        for (int i = 0; i < res.length; i++) {
    
    
            res[i] = list.get(i);
        }

        return res;
    }
}