The algorithm works because if a vertex has no successors, it must be the last one in the topological ordering.

算法能够执行是因为，如果一个顶点没有后继，那么它肯定是拓扑序列中的最后一个。

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Studies have shown that the Hilbert curve preserves very well the topological information, even better than the Z-ordering.

研究表明，Hilbert曲线可以很好地保存拓扑信息，甚至优于Z顺序曲线。

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