题解 | #从上往下打印二叉树#

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<Integer> resultList = new ArrayList<Integer>();
        Deque<TreeNode> deque = new LinkedList<TreeNode>();
        if(root == null){
            return resultList;
        }
        deque.offer(root);

        while(!deque.isEmpty()){
            TreeNode node = deque.poll();
            resultList.add(node.val);
            if(node.left != null){
                deque.offer(node.left);
            }
            if(node.right != null){
                deque.offer(node.right);
            }
        
        }
        return resultList;
    }
}