Problem category filters are not handled as unions

[breezewish]

When "动态规划" and "搜索" are both selected, there is nothing left in problem list.

[Ivecia]

The categories are not right sometimes.

Admin should fix it, but there're too many subjects.

Why not add a funcation about voting the categories when users AC the problems and sort them automatically.

[twd2]

@SkyLionel New PR, please.

[ksyx]

@skylionel
That's so luogu. It seems so automatic, but it needs more management because some user will abuse that(such as select all), so I think it's not so good. But this isn't meanning we can't have this feature.
And I think these categories are set when a problem is created and I think these category may be the algorithm of standard program. We think that's incorrect maybe because we are just using another algorithm.
In the end, i have a idea, how about get them from the solution? In many cases, search some keywords from it and add them to user-contributed category.
In my mind, solutions in Vijos has a regular pattern.
Easy problems has more solutions with description and Hard problems has more solutions which only contain codes.
So, how about using these regular pattern to automaticlly generate categories?

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Keyword Searching:
For each solution, search description keywords(such as 动态规划，搜索，最短路) and code keywords(such as DP,search/DFS/BFS,SPFA/Floyd/Kruskal)(Case insensitive)
Thanks for reading.

[breezewish]

@SkyLionel That's a good idea and we will consider. However currently we are not going to implement it since our dev resource is very limited.

@ksyx Cool idea. I am just afraid that it will not work in many situations. We will take a look at this in future.