Given a list of pairs, each containing the name of a node and the name of its parent, respectively, this builds the full tree as a nested dictionary. Note that the parent of the root node is specified by None.

tree_pairs.py

def build(pairs, root=None):
    tree = {}
    for (child, parent) in pairs:
        if parent == root:
            tree[child] = build(filter(lambda x: x != (child, parent), pairs), child)
    return tree

print build([("H", "G"), ("F", "G"), ("G", "D"), ("E", "D"), ("A", "E"), ("B", "C"), ("C", "E"), ("D", None)])