剑指 Offer 32 - II. 从上到下打印二叉树 II(层序)

    public List<List<Integer>> levelOrder(TreeNode root) {
    
    
        if (root == null) return new ArrayList<>();
        List<List<Integer>> res = new ArrayList<>();
        Deque<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()){
    
    
            int size = queue.size();
            ArrayList<Integer> list = new ArrayList<>();
            for (int i = 0; i < size; i++) {
    
    
                TreeNode node = queue.pop();
                list.add(node.val);
                if (node.left != null) queue.offer(node.left);
                if (node.right != null) queue.offer(node.right);
            }
            res.add(list);
        }
        return res;
    }