题目描述

从上往下打印出二叉树的每个节点，同层节点从左至右打印。

解题思路

题意其实就是二叉树的层次遍历,用队列来实现

public ArrayList<Integer> PrintFromTopToBottom2(TreeNode root) {
    ArrayList<Integer> result = new ArrayList<Integer>();
    ArrayList<TreeNode> helpList = null; //辅助队列
    TreeNode tmp = null;
    if(root == null){
        return result;
    }else{
        helpList = new ArrayList<TreeNode>();
        helpList.add(root);
        while (!helpList.isEmpty()){
            //队列不为空，证明有结点没遍历完
            tmp = helpList.remove(0);//取出队首
            result.add(tmp.val);
            if(tmp.left != null)
                helpList.add(tmp.left);
            if(tmp.right != null)
                helpList.add(tmp.right);
        }
        return result;
    }
}