documentclass{article}
usepackage{tikz}
usetikzlibrary{calc,arrows,positioning}
begin{document}
tikzset{
treenode/.style = {align=center},
c/.style = {treenode,circle,draw=black,minimum width=1.5em,minimum height=1.5em,text centered,font=footnotesize},
r/.style = {treenode,rectangle,draw=black,minimum width=1.5em,minimum height=1.5em,text centered,font=footnotesize},
level 1/.style={sibling distance=60mm},
level 2/.style={sibling distance=30mm},
level 3/.style={sibling distance=15mm},
level 4/.style={sibling distance=7mm}
}
begin{tikzpicture}[->,>=stealth',thick]
node[c] {a}
    child{ node[c] {b}
            child{ node[c] {c}
                    child{ node[c] {d}
                        child{ node[r] {0}}
                        child{ node[r] {0}}
                    }
                    child{ node[c] {d}
                        child{ node[r] {0}}
                        child{ node[r] {1}}
                    }
            }
            child{ node [c] {c}
                    child{ node[c] {d}
                        child{ node[r] {0}}
                        child{ node[r] {0}}
                    }
                    child{ node[c] {d}
                        child{ node[r] {0}}
                        child{ node[r] {1}}
                    }
            }
    }
    child{ node[c] {b}
            child{ node [c] {c}
                    child{ node[c] {d}
                        child{ node[r] {0}}
                        child{ node[r] {0}}
                    }
                    child{ node[c] {d}
                        child{ node[r] {0}}
                        child{ node[r] {1}}
                    }
            }
            child{ node [c] {c}
                    child{ node[c] {d}
                        child{ node[r] {1}}
                        child{ node[r] {1}}
                    }
                    child{ node[c] {d}
                        child{ node[r] {1}}
                        child{ node[r] {1}}
                    }
            }
    }
;
end{tikzpicture}
end{document}