Question: I want to draw complete binary tree with some portion highlighted. I am able to draw the complete binary tree, but not able to highlight the specified portion.

I want to draw the digram given below:

[![enter image description here](https://i.stack.imgur.com/mj9OZ.jpg)](https://i.stack.imgur.com/mj9OZ.jpg)

Till now I'm able to do this much:

```
\documentclass[border=2pt]{standalone}
\usepackage{forest}
\begin{document}
\begin{forest}
for tree={l+=0.07cm} % increase level distance
  [1
    [2[4][5]]
    [3[6][7]]
  ]
\end{forest}
\end{document}

```

Answer: 
I think, in theory, that this should be possible by adding 

```
tikz={\node[draw,circle,red,fit=()(!1)(!2)}

```

to the root of the node that you want to circle. Here `()` refers to the node and `(!1)` and `(!2)` its children. Unfortunately, this doesn't quite work because it gives:

[![enter image description here](https://i.stack.imgur.com/KCO35.png)](https://i.stack.imgur.com/KCO35.png)

So, in practice, it looks better with a little extra tweaking:

[![enter image description here](https://i.stack.imgur.com/sDvdp.png)](https://i.stack.imgur.com/sDvdp.png)

See section 2.3 of the manual for more details.

Here is the full code:

```
\documentclass[border=4pt]{standalone}
\usepackage{forest}
\begin{document}

  \begin{forest}
    for tree={l+=0.08cm} % increase level distance
      [1
        [2,tikz={\node[draw,circle,red,fit=()(!1),inner sep=0mm,xshift=1mm]{};}
           [4][5]
        ]
        [3
           [6][7]
        ]
      ]
  \end{forest}

\end{document}

```

Another option, which might be preferable, is to use

```
tikz={\node[draw,circle,red,fit=()(!1)(!2),inner sep=0mm]{};}

```

giving

[![enter image description here](https://i.stack.imgur.com/grkG9.png)](https://i.stack.imgur.com/grkG9.png)
Here is another solution to use the [`istgame`](https://ctan.org/pkg/istgame) package. Since the `istgame` environment is (almost) the same as the `tikzpicture` environment, you can use any `TikZ` macros in the `istgame` environment.

[![enter image description here](https://i.stack.imgur.com/04UeA.jpg)](https://i.stack.imgur.com/04UeA.jpg)

```
\documentclass{standalone}

\usepackage{istgame}
\usepackage{makecell}

\begin{document}

\begin{istgame}
% tree (istgame macros)
\xtdistance{15mm}{30mm}
\istroot(1){1} \istb \istb \endist
\xtdistance{15mm}{15mm}
\istroot(2)(1-1)<180>{2}
  \istb*{}{4} \istb*{}{5} \endist
\istroot(3)(1-2)<0>{3}
  \istb*{}{6} \istb*{}{7} \endist
\xtSubgameOval(2){(2-1)(2-2)}[circle,solid,red,inner sep=10pt]
% comments (tikz macros)
\coordinate (aa) at ([shift={(-2.5,-.5)}]2);
\draw [->] (aa) node [left] {Block} -- + (1,0);
\coordinate (bb) at ([shift={(2,.3)}]3);
\draw [->] (bb) node [right] {\makecell[l]{Complete \\ binary \\ tree}} -- + (-1,0);
\end{istgame}
\end{document}

```