def DFS(G):
    def DFSVisit(u):
        for v in G[u]:
            if parents[v]==-1:
                parents[v]=u
                DFSVisit(v)
    parents=[-1]*len(G)
    for v in range(len(G)):
        if parents[v]==-1:
            parents[v]=None
            DFSVisit(v)
    return parents
G = [[1,2],[0,2,3],[3,1,0],[]]
print( DFS(G) )