Just a quick response to the first answer (don't get this solution seriously). 

    currentLabel = 0
    f = {}
    def DFS_Loop(graph):
        checked = []
        global currentLabel
        currentLabel = len(graph)
        for edge in graph.keys():
            if edge not in checked :
                checked = DFS(graph,edge, checked)
    
    def DFS(graph, start, path=[]):
        path = path + [start]
        global currentLabel
        global f 
        for edge in graph[start]:
            if edge not in path:
                path = DFS(graph, edge, path)
     
        f[currentLabel] =  start
        currentLabel = currentLabel - 1
        return path
    
    graph = {
        1: [2,11],
        2: [3],
        11: [12],
        12: [13],
        3: [],
        13:[]
        }
    DFS_Loop(graph)
    print f