Android Studio Cannot resolve symbol [How to Solve]

Problem:
Previously the project is normal, one day suddenly appeared red exception, the code is prompted in many places “Cannot resolve symbol xxxx” Log

Analysis:
The problem is caused by the failure of the three-party library dependency, the guide package can not be found

Solution (you can follow the following process to try):
1, Build->Clean Project and then Build->Rebuild Project to see if it is OK, not continue
2,File->Sync Project With Gradle Files to see if it’s OK, if not, continue
3,File->Invalidate Caches/Restart (check Clear Cache or Files) to see if it’s OK, or not.
4, close AS, manually enter the project to delete the .gradle and .idea files, reopen AS build to see if it is OK, not continue
5, downgrade the AS version, keep the settings and uninstall the current version, download and install the old version without problems before

The above five steps will basically solve the problem, which step is OK, just follow the order.
My problem is to the fifth step, from AS Bumblebee directly downgrade to AS4.2.1 before the problem is solved, the middle of the Arctic fox also have problems, specifically ignore.

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