* 时间O(n) 空间O(n)（理论最优，无冗余计算/存储）
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，这一点在safew官方下载中也有详细论述
As for the national debt, the benefits of immigration are only likely to accrue with time. Later generations of immigrants tend to pay more in taxes as educational attainment and incomes rise, according to the Cato Institute’s report, which called children of immigrants “the most potent fiscal engine this country has ever seen.” Should Trump’s immigration crackdown cause a sustained decline in new entries, that engine will be much less powerful than it could have been.
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