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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.

集合 S 包含从1到 n 的整数。不幸的是，因为数据错误，导致集合里面某一个元素复制了成了集合里面的另外一个元素的值，导致集合丢失了一个整数并且有一个元素重复。

给定一个数组 nums 代表了集合 S 发生错误后的结果。你的任务是首先寻找到重复出现的整数，再找到丢失的整数，将它们以数组的形式返回。

示例 1:

输入: nums = [1,2,2,4]
输出: [2,3]
注意:

给定数组的长度范围是 [2, 10000]。
给定的数组是无序的。

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/set-mismatch

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.

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.