从上往下打印二叉树

import java.util.ArrayList;
import java.util.Collections;
/**
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<TreeNode> queue = new ArrayList<TreeNode>();
        ArrayList<Integer> list = new ArrayList<>();
        if(root==null)return list;
        queue.add(root);
        while(!queue.isEmpty()) {
            TreeNode temp = queue.remove(0);
            list.add(temp.val);
            if(temp.left!=null)queue.add(temp.left);
            if(temp.right!=null)queue.add(temp.right);
        }
        return list;
    }
}