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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

Problem:
OSSU references a wide variety of curricula and guidelines, making course evaluation more difficult.

Duration:
2020/04/20

Background:
OSSU’s tagline is simple and clear. To provide a Path to a free self-taught education in Computer Science! Our summary further clarifies that `It is designed according to the degree requirements of undergraduate computer science major

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.

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 UI of book cards is not that great currently. Lot of improvements can be made in terms of how they look. For example how the category is displayed currently in the card is not pleasant and can be improved. Feel free to make any changes that you think improve the design of cards.

See https://github.com/hackerkid/Mind-Expanding-Books/tree/master/app for details on setting up the development

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.