Binary Tree Zigzag Level Order Traversal

java很给力逆序用了Colletcions.reverse();
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
       
        ArrayList<ArrayList<Integer>> ret=new ArrayList<ArrayList<Integer>>();
        if(root==null) return (List)ret;
        Queue<TreeNode> q=new LinkedList<TreeNode>();
        boolean flag=true;
        q.offer(root);
        while(!q.isEmpty())
        {
            int len=q.size();
            ArrayList<Integer> temp=new ArrayList<Integer>();
            for(int i=0;i<len;i++)
            {
                TreeNode t=q.poll();
                
                temp.add(t.val);
                
                if(t.left!=null)q.offer(t.left);
                if(t.right!=null) q.offer(t.right);
            }
            if(!flag) Collections.reverse(temp);
            ret.add(temp);
            
            flag=!flag;
                
            
            
        
        }
        
        return (List)ret;
    }
}