题解 | #从上往下打印二叉树#

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) {
        // 判断当root为空的情况
        ArrayList<Integer> list = new ArrayList<Integer>();
        if(root == null){
            return list;
        }

        // root不为空
        ArrayList<TreeNode> tList = new ArrayList<TreeNode>();
        tList.add(root);
        while(tList.size() != 0){
            TreeNode temp = tList.remove(0); // 删除指定索引位置的元素 arraylist.remove(int index)
            list.add(temp.val);
            if(temp.left != null){
                tList.add(temp.left);
            }
            if(temp.right != null){
                tList.add(temp.right);
            }
        }
        return list;
    }
}