题解 | #从上往下打印二叉树#

牛客困难简单题有点混乱啊！简单层序遍历。

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) {
        Queue<TreeNode> queue = new LinkedList<>();
        ArrayList<Integer> we = new ArrayList<>();
        if(root == null){
            return we;
        }
        queue.offer(root);
        while(!queue.isEmpty()){
            root = queue.poll();
            we.add(root.val);
            if(root.left!=null){
                queue.offer(root.left);
            }
            if(root.right !=null){
                queue.offer(root.right);
            }
        }
        return we;
    }
}