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) {
        if (root == null) {
            return new ArrayList<>();
        }
        List<Integer> list = new ArrayList<>();

        Queue<TreeNode> queue = new ArrayDeque<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode c = queue.poll();
            list.add(c.val);
            if (c.left != null) {
                queue.offer(c.left);
            }
            if (c.right != null) {
                queue.offer(c.right);
            }
        }

        return (ArrayList<Integer>) list;
    }
}