digraph dtree {
    node [fontsize=14, shape=box];

    0;
    1.1;
    1.2;
    2.1 [style=filled, fillcolor=grey89];
    2.2 [style=filled, fillcolor=grey89];
    2.3 [style=filled, fillcolor=grey89];
    2.4 [style=filled, fillcolor=grey89];
    3.1 [style=filled, fillcolor=grey49];
    3.2 [style=filled, fillcolor=grey49];
    3.3 [style=filled, fillcolor=grey49];

    0 -> 1.1
    0 -> 2.1
    0 -> 3.1
    1.1 -> 1.2
    2.1 -> 2.2
    2.2 -> 2.3
    1.2 -> 2.3

    3.1 -> 3.2
    1.1 -> 3.2
    3.2 -> 3.3
    2.2 -> 3.3

    2.3 -> 2.4
    3.3 -> 2.4
}