/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public void reverse(LinkedList<Integer> list)
     {
         for(int i=0;i<list.size()/2;i++)
         {
             int temp=list.get(i);
             list.set(i, list.get(list.size()-1-i));
             list.set(list.size()-1-i, temp);
         }
     }

    public List<List<Integer>> printFromTopToBottom(TreeNode root) {
        List<List<Integer>> arr=new ArrayList<List<Integer>>();
        if(root==null)
            return arr;
        LinkedList<Integer> list=new LinkedList<Integer>();
        LinkedList<TreeNode> q=new LinkedList<TreeNode>();
        q.add(root);
        TreeNode last=root;
        boolean flag=false;
        int level=1;
        while(!q.isEmpty())
        {
            TreeNode node=q.poll(); 
            list.add(node.val);
            if(last==node)
            {
                flag=true;
                if(level++%2==0)//相对于上一题加上偶数行的逆置即可
                    reverse(list);
                arr.add(list);
                list=new LinkedList<Integer>();
            }
            if(node.left!=null)
                q.add(node.left);
            if(node.right!=null)
                q.add(node.right);
            if(flag)
            {
                last=q.peekLast();
                flag=false;
            }

        }
        return arr;
    }
}