JZ59-按之字形顺序打印二叉树

public class Solution {
    ArrayList<ArrayList<Integer>> ret = new ArrayList<>();
    public ArrayList<ArrayList<Integer>> Print(TreeNode pRoot) {
        if(pRoot==null){
            return ret;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(pRoot);
        boolean flag = false;
        while(!queue.isEmpty()){
            ArrayList<Integer> list = new ArrayList<>();
            int size = queue.size();
            while(size>0){
                TreeNode temp = queue.poll();
                list.add(temp.val);
                if(temp.left!=null){
                    queue.add(temp.left);
                }
                if(temp.right!=null){
                    queue.add(temp.right);
                }
                size--;
            }
            if(flag){
                Collections.reverse(list);
            }
            flag= !flag;
            if(list.size()!=0){
                ret.add(list);
            }
        }
        return ret;
    }
}