逐层打印二叉树

import java.util.*;
import java.util.ArrayList;
/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

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

    }

}
*/
public class Solution {
    
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
    ArrayList res = new ArrayList<Integer>();
    if(root == null) return res; 
    Queue<TreeNode> q = new ArrayDeque<TreeNode>(); //队列这里用到ArrayDequeue是容量可变数组
    q.add(root);
    while(!q.isEmpty()){ // 逐层弹出节点
       TreeNode poll = q.poll();
       res.add(poll.val);
       if(poll.left != null) q.add(poll.left);
       if(poll.right != null) q.add(poll.right);
    }
    return res;
    }

}

因为需要逐层便利且是从左及右，所以方可选择队列这个数据结构，因为他会按顺序输出数据且按顺序放入子节点