按之字形顺序打印二叉树

class Solution{
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;
    public TreeNode(int val) {
        this.val = val;
    }
}
    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
    ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();  
    if(pRoot==null)//边界条件
    return result;
    ArrayList<TreeNode> ele = new ArrayList<TreeNode>();//保存每一层的树节点
    ele.add(pRoot);
    boolean flag = true;//标记位
    while(!ele.isEmpty()){
    ArrayList<Integer> temp = new ArrayList<Integer>();//每一层树节点的值
    int len = ele.size();
    for(int i = 0;i<len;i++){
    TreeNode node = ele.get(0);
    if(node.left!=null)
    ele.add(node.left);
    if(node.right!=null)
    ele.add(node.right);
    if(flag)
    temp.add(node.val);//尾部追加类似于队列
    else
    temp.add(0,node.val);//从头部添加，类似于栈
    ele.remove(0);
    } 
    flag ^=true;//改变放入方向
    result.add(temp);
    }
    return result;
    }
}