集合中二叉树程序遍历

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> ret = new ArrayList<>();
        if(root == null) return ret;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()){
            List<Integer> list = new ArrayList<>();
            int size = queue.size();
            while(size != 0){
                TreeNode cur = queue.poll();
                list.add(cur.val);
                if(cur.left != null) queue.offer(cur.left);
                if(cur.right != null) queue.offer(cur.right);
                size--;
            }
            ret.add(list);
        }
        return ret;
    }
}