轻车熟路了

练速度

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> result = new ArrayList<>();
        if(root==null) return result;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()){
            int len = queue.size();
            for(int i=0;i<len;i++){
                TreeNode node = queue.poll();
                result.add(node.val);
                if(node.left!=null) queue.offer(node.left);
                if(node.right!=null) queue.offer(node.right);
            }
        }
        return result;

    }
}