3] Implement the iterative version of depth-first search. Here, for simplicity, we use visited as a list to store all visited vertices in order. In Python, you can use list as a stack. It supports pop () function, and append () on a list is just equivalent to push an element to a stack. def DFS iterative (graph, source): ### START YOUR CODE ### 0.5s stack = None # Initialize the stack properly visited = [] # Intialize to an empty list while None: # Specify the loop range pass # Add necessary code ### END YOUR CODE ### return visited # Do not change the test code here print () dg = Graph (directed=True) 0.3s dg.add_edge('u', 'v') dg.add_edge('u', 'w') dg.add_edge('w', 'x') dg.add_edge('v', 'x'). dg.add_edge('v', 'y') dg.add_edge('x', 'v') dg.add_edge('x', 'y'). print('DFS on \'u\':') visited print (visited) DFS_iterative (dg, 'u') print('DFS on \'w\':') visited DFS_iterative (dg, 'w') print (visited)