leetcode 655. 输出二叉树 【时间击败100.00%】 【内存击败96.49%】

 public List<List<String>> printTree(TreeNode root) {
        ArrayList<List<String>> al = new ArrayList<>();
        if (root == null) return al;
        if (root.left == null && root.right == null) {
            al.add(new ArrayList<>());
            al.get(0).add(String.valueOf(root.val));
            return al;
        }

        int high = dep(root, 1);
        String[][] ans = new String[(int) Math.pow(2, high)][high + 1];
        add(root, (int) Math.pow(2, high - 1), 1, (int) Math.pow(2, high - 2), ans);
        for (int i = 1; i <= high; i++) {
            al.add(new ArrayList<>());
            for (int j = 1; j < ans.length; j++)
                al.get(i - 1).add(ans[j][i] == null ? "" : ans[j][i]);
        }
        return al;
    }

    public void add(TreeNode cur, int x, int y, int diff, String[][] ans) {
        if (cur == null) return;
        ans[x][y] = cur.val + "";
        add(cur.left, x - diff, y + 1, diff / 2, ans);
        add(cur.right, x + diff, y + 1, diff / 2, ans);
    }


    public int dep(TreeNode cur, int dep) {
        if (cur == null) return dep - 1;
        return Math.max(dep(cur.left, dep + 1), dep(cur.right, dep + 1));
    }