class Node(object):
    def __init__(self, value, dependencies):
        self.value = value
        self.dependencies = dependencies

def output_pattern(graph_root):
    stack = [graph_root]
    output = []

    while len(stack):
        # print 'stack: %s' % stack

        curr_node = stack.pop()

        output.append(curr_node.value)

        for dependency in curr_node.dependencies:
            stack.append(dependency)

    print ' '.join(output)


node_f = Node('f', [])
node_e = Node('e', [])
node_b = Node('b', [node_f, node_e])
node_d = Node('d', [])
node_a = Node('a', [node_b, node_d])

output_pattern(node_a)