def DFS(G):
    def DFSVisit(u):
        visited[u]=True
        for v in G[u]:
            if not visited[v]:
                visited[v]=True
                parents[v]=u
                DFSVisit(v)
    parents=[None]*len(G)
    visited=[False]*len(G)
    for v in range(len(G)):
        if not visited[v]:
            DFSVisit(v)
    return parents
G = [[1,2],[0,2,3],[3,1,0],[1,2]]
print(DFS(G))