human
Solve the following python programming problem given the following code:
# Definition for a Directed graph node
# class DirectedGraphNode:
#     def __init__(self, x):
#         self.label = x
#         self.neighbors = []

class Solution:
    """
    @param graph: A list of Directed graph node
    @return: A list of graph nodes in topological order.
    """
    def topSort(self, graph):
        # write your code here
        dic = {}
        for node in graph:
            dic[node] = 0
        
        for node in graph:
            for n in node.neighbors:
                dic[n] += 1
        
        order = []
        for node in graph:
            if dic[node] == 0:
                order.append(node)
        
        queue = order[:]   ## right way to copy a list!!!
        while queue:
            node = queue.pop(0)
            for n in node.neighbors:
                dic[n] -= 1
                if dic[n] == 0:
                    order.append(n)
                    queue.append(n)
        
        return order if len(order) == len(dic) else None
            
Given the above code, generate some markdown instructions for it.
assistant
#Here is the Python code you requested.

Topological Sorting

Given an directed graph, a topological order of the graph nodes is defined as follow:
For each directed edge A -> B in graph, A must before B in the order list.
The first node in the order can be any node in the graph with no nodes direct to it.
Find any topological order for the given graph.
http://www.lintcode.com/en/problem/topological-sorting/