def shortest_path(source, target):
"""
Returns the shortest list of (movie_id, person_id) pairs
that connect the source to the target.
If no possible path, returns None.
"""
global num_explored
# Initialize starting node and frontier
start = Node(state=source, parent=None, action=None)
frontier = QueueFrontier()
frontier.add(start)
# initialize set for explored nodes
explored = set()
# Loop until solution found
while True:
# If nothing left in frontier, then there is no connection
if frontier.empty():
return None
# Take the node from the frontier
node = frontier.remove()
num_explored += 1
# Mark node as explored
explored.add(node.get_state())
# Add neighbors to the frontier
for action, state in neighbors_for_person(node.get_state()):
if not frontier.contains_state(state) and state not in explored:
child = Node(state, node, action)
frontier.add(child)
# If the node is the target, then there is connection
if child.get_state() == target:
connections = []
while child.get_parent() is not None:
connections.append((child.get_action(), child.get_state()))
child = child.get_parent()
connections.reverse()
return connections
