solved 'Journey to the Moon' on hackerrank https://www.hackerrank.com/challenges/journey-to-the-moon

connected_component_sizes.py

def get_connected_component_sizes(graph, node_num):
  """graph: Adjacency list for the graph.
     node_num: Number of nodes."""
  visited = [False] * node_num
  component_sizes = []
  
  for v in xrange(node_num):
    if not visited[v]:
      # DFS using stack.
      stack, sz = [v], 0
      visited[v] = True
      while stack:
        top_node = stack.pop()
        sz += 1
        for neighbor in graph[top_node]:
          if not visited[neighbor]:
            stack.append(neighbor)
            visited[neighbor] = True
      component_sizes.append(sz)

  return component_sizes  


if __name__ == '__main__':
  node_num, edge_num = map(int,raw_input().split())
  adjacency_list = [list() for _ in xrange(node_num)]

  for _ in xrange(edge_num):
    src, dest = map(int, raw_input().split())
    # Note this is an undirected graph.
    adjacency_list[src].append(dest)
    adjacency_list[dest].append(src)

  component_sizes = get_connected_component_sizes(
      adjacency_list, node_num)

  pair_num = 0
  for sz in component_sizes:
    pair_num += sz * (node_num - sz)
  # Note each component has been calculated twice.
  print pair_num / 2