Add only arrow to some edges of the tree

Question

I want to draw a picture like following:

\begin{tikzpicture} [ level 1/.style={sibling distance=30mm}, 
level 2/.style={sibling distance=15mm},
level 3/.style={sibling distance=7mm},
triangle/.style = {fill=blue!20, regular polygon, regular polygon sides=3},
node rotated/.style = {rotate=180},
border rotated/.style = {shape border rotate=180}]
\node[triangle] {1.5} 
    child {node[circle, draw, fill=blue!20] {1.5} 
        child {node[triangle, border rotated] {2} 
            child {node {2}}
            child {node {2}}}
        child {node[triangle, border rotated] {1}
            child {node {1}}
            child {node {2}}}}
    child {node[circle, draw, fill=blue!20] {-0.5}
        child {node[triangle, border rotated] {0}
            child {node {0}}
            child {node {2}}}
        child {node[triangle, border rotated] {-1}
            child {node {-1}}
            child {node {0}}}};
\end{tikzpicture}

This is my code. I've viewed this question TikZ: turning edges into arrows using "child" notation However when I add edge to parent, it will make whole tree's edge with arrow. I don't know how to make only one edge to red and arrow and don't change others. Thank you for your help.

Answer 1

You can always declare an edge from parent style on desired node and revert it in descendant branches

\documentclass[border=2mm,tikz]{standalone}
\usetikzlibrary{shapes.geometric}

\begin{document}
\begin{tikzpicture} [ level 1/.style={sibling distance=30mm}, 
level 2/.style={sibling distance=15mm},
level 3/.style={sibling distance=7mm},
triangle/.style = {fill=blue!20, regular polygon, regular polygon sides=3},
node rotated/.style = {rotate=180},
border rotated/.style = {shape border rotate=180}]
\node[triangle] {1.5} 
    child {node[circle, draw, fill=blue!20] {1.5} edge from parent[->, thick, draw=red]
        child {node[triangle, border rotated] {2} edge from parent[-, thin, draw=black] 
            child {node {2}}
            child {node {2}}}
        child {node[triangle, border rotated] {1} edge from parent[-, thin, draw=black]
            child {node {1}}
            child {node {2}}}}
    child {node[circle, draw, fill=blue!20] {-0.5}
        child {node[triangle, border rotated] {0}
            child {node {0}}
            child {node {2}}}
        child {node[triangle, border rotated] {-1}
            child {node {-1}}
            child {node {0}}}};
\end{tikzpicture}
\end{document}

Answer 2

An answer by using styles:

The output is like @Ignasi's answer output but I used styles and didn't made the arrow thick:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shapes}
\begin{document}
\begin{tikzpicture} [ level 1/.style={sibling distance=30mm}, 
level 2/.style={sibling distance=15mm},
level 3/.style={sibling distance=7mm},
triangle/.style = {fill=blue!20, regular polygon, regular polygon sides=3},
node rotated/.style = {rotate=180},
border rotated/.style = {shape border rotate=180},
mystyle/.style={-latex,red},
mydefault/.style={-,black}
]
\node[triangle] {1.5} 
    child[mystyle]  { node[circle, fill=blue!20,draw=black,text=black]  {1.5}
        child[mydefault] {node[triangle, border rotated] {2} 
            child {node {2}}
            child {node {2}}}
        child[mydefault] {node[triangle, border rotated] {1}
            child {node {1}}
            child {node {2}}}}
    child {node[circle, draw, fill=blue!20] {-0.5}
        child {node[triangle, border rotated] {0}
            child {node {0}}
            child {node {2}}}
        child {node[triangle, border rotated] {-1}
            child {node {-1}}
            child {node {0}}}};
\end{tikzpicture}
\end{document}

PS: I wrote that answer many hours before (but then had some urgent chats about work) and just adding because it is ready.