[LeetCode]Subtree of Another Tree判断一棵树是不是另一棵树的子树

List<String> list = new ArrayList<>();
    public boolean isSubtree(TreeNode s, TreeNode t) {
        helper(s);
        helper(t);
        String tt = list.get(list.size()-1);
        for (int i = 0;i<list.size()-1;i++) {
            if (list.get(i).equals(tt)) return true;
        }
        return false;
    }
    public String helper(TreeNode root)
    {
        StringBuilder cur = new StringBuilder();
        if (root==null)
        {
            cur.append("#");
            return new String(cur);
        }
        cur.append(root.val);
        cur.append(helper(root.left));
        cur.append(helper(root.right));
        String s = new String(cur);
        list.add(s);
        return s;
    }