Added assignment 5 solution file .

assignment_5.py

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+def find_language_characters(ordered_list):
+	# use a dict to represent a graph
+	# key: item, value: neighbors
+	graph = {}
+	graph = fill_graph(False, graph, ordered_list)
+	graph = fill_graph(True, graph, ordered_list)
+
+	# map all nodes to visited or not flag
+	node_visited = {}
+	for item in graph.keys():
+		if item not in node_visited.keys():
+			node_visited[item] = False
+
+		for val in graph[item]:
+			if val not in node_visited.keys():
+				node_visited[val] = False
+
+	# find predecessors of each node, then sort topollogically
+	predecessors_graph = find_predecessors(graph, node_visited)	
+	return topological_sort(graph, node_visited, predecessors_graph)
+
+def topological_sort(graph, node_visited, predecessors_graph):
+	stack = []
+	final_list = []
+
+	for v in node_visited.keys():
+		if v not in predecessors_graph.keys():
+			stack.append(v)
+			node_visited[v] = True
+	while(len(stack) != 0):
+		v = stack.pop()
+		stack.append(v) # we dont want to remove it yet
+
+		# find out if they're all visited
+		all_vis = True
+		if v in graph.keys() and graph[v] != None:
+			for nv in graph[v]:
+				if node_visited[nv] == False:
+					all_vis = False # no, they're not all visited
+
+		if(all_vis):
+			final_list.append(stack.pop())
+		else:
+			for nv in graph[v]:
+				if node_visited[nv] == False:
+					stack.append(nv)
+					node_visited[nv] = True
+	return final_list[::-1]
+
+def fill_graph(fill, graph, ordered_list):
+	# traverse through the words in sorted order two by two
+	for i in range(1, len(ordered_list)):
+		w1 = ordered_list[i-1]
+		w2 = ordered_list[i]
+		# keep two pointers in each of the words
+		p1 = 0
+		p2 = 0
+
+		# init neighbors
+		while w1[p1] == w2[p2] and p1 < len(w1) and p2 < len(w2): 
+			p1 += 1
+			p2 += 1
+		if fill == False:
+			graph[w1[p1]] = [] # initialize
+		else:
+			graph[w1[p1]].append(w2[p2]) # fill
+	return graph
+
+def find_predecessors(graph, node_visited):
+	predecessors_graph = {}
+	for item in graph.keys():	
+		for value_item in graph[item]:
+			predecessors_graph[value_item] = []
+
+	for item in graph.keys():
+		for value_item in graph[item]:
+			predecessors_graph[value_item].append(item)
+	return predecessors_graph
+
+print(find_language_characters(['ART', 'RAT', 'CAT', 'CAR','art', 'rat', 'cat', 'car']))
+
+
+