102. 二叉树的层次遍历

    // 递归
    private List<List<Integer>> levels = new ArrayList<>();
    public List<List<Integer>> levelOrder(TreeNode root) {
        if (root == null) {
            return levels;
        }
        helper(root, 0);
        return levels;
    }

    private void helper(TreeNode root, int level) {
        if (levels.size() == level) {
            levels.add(new ArrayList<Integer>());
        }
        levels.get(level).add(root.val);
        if (root.left != null) {
            helper(root.left, level + 1);
        }
        if (root.right != null) {
            helper(root.right, level + 1);
        }
    }

    // 迭代
    public List<List<Integer>> levelOrder2(TreeNode root) {
        List<List<Integer>> levels = new ArrayList<>();
        if (root == null) {
            return levels;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        int level = 0;
        while (!queue.isEmpty()) {
            levels.add(new ArrayList<Integer>());

            int levelSize = queue.size();
            for (int i = 0; i < levelSize; i++) {
                TreeNode node = queue.remove();
                levels.get(level).add(node.val);
                if (node.left != null) {
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
            }
            level++;
        }
        return levels;
    }