/android/sdk/build-tools/21.1.2/aapt ' finished with Non-zero exit value 42

I believe a lot of friends will meet

　　Error:Error: com.android.ide.common.process.ProcessException:    org.gradle.process.internal.ExecException: Process ‘command ‘/Users/billy/Library/Android/sdk/build-tools/21.1.2/aapt‘‘ finished with non-zero exit value 42
This problem, find a lot of information, although solved, but still do not know why, the main reason for the problem is because the resource file parsing error.
The common causes are
1. resource File name conflicts
This is a different file name.
2. Reference packet Duplication
Android Studio under the V4 package or V7 package, this time in the Libs to add another package, will report this error, delete the Libs under the duplicate package on the line.
2. File name display format does not correspond to the format of the file itself
This is more difficult to find, re-change the correct file on it.

/android/sdk/build-tools/21.1.2/aapt ' finished with Non-zero exit value 42