力扣练习004---二叉树的层次遍历(102)

    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();
        if (root == null) {
            return result;
        }

        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        result.add(Collections.singletonList(root.val));
        while (!queue.isEmpty()) {
            int queueSize = queue.size();
            List<Integer> list = new ArrayList<>();
            for (int i = 0; i < queueSize; i++) {
                TreeNode node = queue.poll();
                if (node == null) {
                    continue;
                }

                if (node.left != null) {
                    list.add(node.left.val);
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    list.add(node.right.val);
                    queue.offer(node.right);
                }
            }

            if (!list.isEmpty()) {
                result.add(list);
            }
        }

        return result;
    }