cannot resolve symbol R in Android Studio

I had this this issue too. A simple gradlew clean and gradlew build did the trick.

Click on Build->Clean Project and that will perform a gradle clean

In the latest versions of Android Studio, at least for me, the following works:

Tools -> Android -> Sync Project with Gradle Files

In latest Android Studio 3.1.3 (July 2018), Sync Project with Gradle Files is available in main menu bar.

I was using gradle 3.3.0. It was the main culprit. Wasted 6.50 hours from my life. Gradle 3.2.1 removed the error.

classpath com.android.tools.build:gradle:3.2.1

Problem resolved after changing the gradle version. Details history can be found here.