Завершение выполнения кода - Сильно связанные компоненты - Kosaraju

Question

Я нашел в сети код алгоритма Косараджу. Я понимаю общую идею алгоритма и могу следовать объяснениям на абстрактном уровне. Однако, когда дело доходит до самого кода, мой ограниченный опыт программирования не дает мне понять, что происходит. Я приведу полный код ниже. Приветствуется любое объяснение:)

from collections import defaultdict 

#This class represents a directed graph using adjacency list representation 
class Graph: 

    def __init__(self,vertices): 
        self.V= vertices # this is the amount of vertices or nodes the graph has
        self.graph = defaultdict(list) 

    # function to add an edge to graph 
    def addEdge(self,u,v): 
        self.graph[u].append(v)

    # A function used by DFS 
    def DFSUtil(self,v,visited): 
        # Mark the current node as visited and print it 
        visited[v]= True
        print v, 
        #Recur for all the vertices adjacent to this vertex 
        for i in self.graph[v]: 
            if visited[i]==False: 
                self.DFSUtil(i,visited) 


    def fillOrder(self,v,visited, stack): 
        # Mark the current node as visited  
        visited[v]= True
        #Recur for all the vertices adjacent to this vertex 
        for i in self.graph[v]: 
            if visited[i]==False: 
                self.fillOrder(i, visited, stack) 
        stack = stack.append(v) 


    # Function that returns reverse (or transpose) of this graph 
    def getTranspose(self): 
        g = Graph(self.V) 

        # Recur for all the vertices adjacent to this vertex 
        for i in self.graph: 
            for j in self.graph[i]: 
                g.addEdge(j,i) 
        return g 



    # The main function that finds and prints all strongly 
    # connected components 
    def printSCCs(self): 

         stack = [] 
        # Mark all the vertices as not visited (For first DFS) 
        visited =[False]*(self.V) 
        # Fill vertices in stack according to their finishing 
        # times 
        for i in range(self.V): 
            if visited[i]==False: 
                self.fillOrder(i, visited, stack) 

        # Create a reversed graph 
         gr = self.getTranspose() 

         # Mark all the vertices as not visited (For second DFS) 
         visited =[False]*(self.V) 

         # Now process all vertices in order defined by Stack 
         while stack: 
             i = stack.pop() 
             if visited[i]==False: 
                gr.DFSUtil(i, visited) 
                print"" 

# Create a graph given in the above diagram 
g = Graph(5) 
g.addEdge(1, 0) 
g.addEdge(0, 2) 
g.addEdge(2, 1) 
g.addEdge(0, 3) 
g.addEdge(3, 4) 


print ("Following are strongly connected components " +
                           "in given graph") 
g.printSCCs()