class Node:
	def __init__(self, name):
		self.name = name
		self.parent = None
		self.children = []

def pairs_to_trees(pairs):
	nodes = {}
	for parent_name, child_name in pairs:
		parent = nodes.setdefault(parent_name, Node(parent_name))
		child  = nodes.setdefault(child_name,  Node(child_name))
		child.parent = parent
		parent.children.append(child)
	return [node for node in nodes.itervalues() if node.parent is None]

def show(node, depth):
	if depth == 0:
		print node.name
	else:
		print "  " * (depth - 1) + "->" + node.name
	for child in node.children:
		show(child, depth + 1)

def main():
	pairs = [
		('A', 'B'),
		('B', 'C'),
		('C', 'D'),
		('C', 'E'),
		('A', 'F'),
		('D', 'X'),
		('Y', 'Z'),
		('Z', 'C'),
	]

	for tree in pairs_to_trees(pairs):
		show(tree, 0)

if __name__ == '__main__':
	main()