层次遍历二叉树

题目描述：

public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) return res;
        Queue<TreeNode> queue = new LinkedList<>();//队列计入当前层
        queue.offer(root);
        while (!queue.isEmpty()) {
            List<Integer>list=new ArrayList<>();
            int size = queue.size();//遍历队列使用，前size个位当前层
            while(size-->0){
                TreeNode poll = queue.poll();
                list.add(poll.val);
                //左右节点不为空计入队列为下一层
                if (poll.left!=null)queue.offer(poll.left);
                if (poll.right!=null)queue.offer(poll.right);
            }
            res.add(list);
        }
        return res;
    }