解题思路

递归

 List<List<Integer>> levels = new ArrayList<List<Integer>>();
    public void helper(TreeNode node, int level) {
        if (levels.size() == level)
            levels.add(new ArrayList<Integer>());
        // 填充当前层
        levels.get(level).add(node.val);
        // 处理下一层
        if (node.left != null)
            helper(node.left, level + 1);
        if (node.right != null)
            helper(node.right, level + 1);
    }
    public List<List<Integer>> levelOrder(TreeNode root) {
        if (root == null)
            return levels;
        helper(root, 0);
        return levels;
    }

迭代

public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> levels = new ArrayList<List<Integer>>();
        if (root == null) return levels;
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        int level = 0;
        while ( !queue.isEmpty() ) {
            // start the current level
            levels.add(new ArrayList<Integer>());
            // number of elements in the current level
            int level_length = queue.size();
            for(int i = 0; i < level_length; ++i) {
                TreeNode node = queue.remove();
                // fulfill the current level
                levels.get(level).add(node.val);
                // add child nodes of the current level
                // in the queue for the next level
                if (node.left != null) queue.add(node.left);
                if (node.right != null) queue.add(node.right);
            }
            // go to next level
            level++;
        }
        return levels;
    }