How to get the following planar binary trees diagram in Latex?

Question

I want to get the above diagram in Latex, but I don't know how to edit it. Any help will be appreciated. I know I had better post my effort here, but I really can not get it by tikz. I find an example written by Stefan Kottwitz. I try to modify his example to get what I want. But I am failed.

% A simple Tree
% Author: Stefan Kottwitz
% https://www.packtpub.com/hardware-and-creative/latex-cookbook
\documentclass[border=10pt]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[sibling distance=10em,
  every node/.style = {shape=rectangle, rounded corners,
    draw, align=center,
    top color=white, bottom color=blue!20}]]
  \node {Formulas}
    child { node {single-line} }
    child { node {multi-line}
      child { node {aligned at}
        child { node {relation sign} }
        child { node {several places} }
        child { node {center} } }
      child { node {first left,\\centered,\\last right} } };
\end{tikzpicture}
\end{document}