从上往下打印二叉树【Java】

题目：

从上往下打印出二叉树的每个节点，同层节点从左至右打印。

很明显，这道题目就是树的顺序遍历(个人喜欢这么说）也叫层次遍历

首先看下层次遍历的特点：

当然，并不只有这一种解法，更多的解法大家也可以在评论留言呀~

 

上代码

import java.util.ArrayList;
import java.util.Queue;
import java.util.concurrent.LinkedBlockingQueue;

public class Solution {

    public static void main(String[] args) {
        Solution solution = new Solution();
        TreeNode root = new TreeNode(1);
        TreeNode right = new TreeNode(3);
        TreeNode left = new TreeNode(2);
        TreeNode left1 = new TreeNode(4);
        TreeNode right2 = new TreeNode(5);
        root.left = left;
        root.right = right;
        left.right = right2;
        right.left = left1;
        solution.PrintFromTopToBottom(root).forEach(System.out::print);
    }

    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> list = new ArrayList<>();
        if(root == null){
            return list;
        }
        // ---------------------------核心代码-------------------------
        Queue<TreeNode> queue = new LinkedBlockingQueue<>();
        queue.add(root);
        while (!queue.isEmpty()){
            TreeNode peek = queue.remove();
            list.add(peek.val);
            // 利用队列先进先出的特性，依次将节点放入队列中，供后续消费
            if(peek.left != null){
                queue.add(peek.left);
            }
            if(peek.right != null){
                queue.add(peek.right);
            }
        }
        // -----------------------------------------------------------
        return list;
    }

}

class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;
    }
}