binary tree zigzag level order traversal

 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class  {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
        if(root==null) return res;
        queue.offer(root);
        boolean change = true;
        while(!queue.isEmpty()){
            int size = queue.size();
            List<Integer> sub = new ArrayList<Integer>();
            for(int i=0;i<size;i++){
                TreeNode node = queue.poll();
                if(change) {
                    sub.add(node.val);
                } else {
                    sub.add(0,node.val);
                }
                if(node.left!=null) queue.offer(node.left);
                if(node.right!=null) queue.offer(node.right);
            }
            res.add(sub);
            change = !change;
        }
        return res;
    }
}