从上往下打印二叉树

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