def topsort(G):
    count = dict((u,0) for u in G)
    for u in G:
        for v in G[u]:
            count[v] +=1
    Q = [u for u in G if count[u] == 0]
    S = []
    while Q:
        u = Q.pop()
        S.append(u)
        for v in G[u]:
            count[v]-=1
            if count[v]==0:
                Q.append(v)
    return S