133. Clone Graph

traverse graph (Breadth First Search)
copy using HashMap 

# Definition for a undirected graph node
# class UndirectedGraphNode(object):
#     def __init__(self, x):
#         self.label = x
#         self.neighbors = []

class Solution(object):
    def cloneGraph(self, node):
        """
        :type node: UndirectedGraphNode
        :rtype: UndirectedGraphNode
        """
        if node is None: 
            return None
        #if graph is not empty, make a copy of the first node 
        cloned_node=UndirectedGraphNode(node.label)
        #create a hashmap with key=node in the original graph,value=cloned node 
        cloned={node:cloned_node}
        #store the next level of nodes to be visited  
        queue=[node]
        
        while queue: 
            current=queue.pop()
            #loop through the neighbours of the current node 
            for neighbor in current.neighbors:
                #if the neighbor node hasn't been cloned before, make a clone 
                if neighbor not in cloned: 
                    queue.append(neighbor)
                    cloned_neighbor=UndirectedGraphNode(neighbor.label)
                    cloned[neighbor]=cloned_neighbor
                #attach the created neighbors to the current graph node 
                cloned[current].neighbors.append(cloned[neighbor])
        return cloned[node]