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What is this issue about?

• D

https://github.com/trekhleb/javascript-algorithms/blob/master/src/algorithms/sets/cartesian-product/cartesianProduct.js#L8

I think it makes more sense to return an empty array

// invalid input return null or throw an error
if (!setA || !setB) {
    return null;
}
if (!setA.length || !setB.length) {
    return []; 
}
// or do nothing since the for loop will not trigger anyway

Histories make men wise.

The link for the course above is dead and should be replaced by https://cs.brown.edu/courses/cs123/index.shtml. I'll create a pull request to address this issue as well.

一个班级60%喜欢足球，70%喜欢篮球，80%喜欢排球，问：三种球都喜欢占比最大可能有多少，最小可能有多少？即求三种球都喜欢的比例范围

Users should be able to sort books on basis of rating, year published, title etc.

See https://github.com/hackerkid/Mind-Expanding-Books/tree/master/app to setup development environment

Some possible suggestions are - sites which teach algorithms, sites which interactively visualize different algorithms. Also, notebooks/question pages of ACM themed contests can be added.