from collections import deque
def topologicalOrdering(g):
    
    # find indegrees of each node
    out = reduce(lambda a,b: a+b, g.values())
    indegrees = {}
    for node in set(out + g.keys()):
        if node not in out:
            indegrees[node] = 0
        else:
            indegrees[node] = out.count(node)

    # collect set of all nodes in graph with zero indegrees
    candidates = deque()
    for node in indegrees:
        if indegrees[node] == 0:
            candidates.appendleft(node)

    L = [] # list for order of nodes
    
    # choose zero indegree node and remove it from graph
    while candidates:
        a = candidates.pop()
        L.append(a)
        if a in g:
            for b in g[a]:
                indegrees[b] -= 1
                if indegrees[b] == 0:
                    candidates.appendleft(b)           
    
    #if graph has edges that have not been removed/if there is a cycle
    for node in indegrees:
        if indegrees[node] != 0:
            return "the input graph is not a DAG"
    else:
        return L