题解 | #从上往下打印二叉树#

import java.util.ArrayList;
import java.util.*;
/**
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> res = new ArrayList<>();
        if (null == root) {
            return res;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        TreeNode node = root;
        queue.add(node);
        while (!queue.isEmpty()) {
            node = queue.poll();
            res.add(node.val);
            if (null != node.left) {
                queue.add(node.left);
            }
            if (null != node.right) {
                queue.add(node.right);
            }
        }
        return res;
    }
}