问"cordova build android“中的错误

EN

FAILURE:Build failed with an exception. 
*Where:
Script'E\cordva\helloworld\platforms\android\CordovaLib\cordova.gradle' line:117
*What went wrong:
A problem occurred evaluating root project 'android'。
>Unable to determine Android SDK directory.

    An exception occurred while trying to find the Android build tools.
    Failed to notify ProjectEvaluationListener.afterEvaluate(), but primary configuration failure takes precedence.
    java.lang.IllegalStateException: buildToolsVersion is not specified.
        at com.google.common.base.Preconditions.checkState(Preconditions.java:173)
        at com.android.build.gradle.BasePlugin.createAndroidTasks(BasePlugin.java:645)
        at com.android.build.gradle.BasePlugin$10.call(BasePlugin.java:608)
        at com.android.build.gradle.BasePlugin$10.call(BasePlugin.java:605)
        at com.android.builder.profile.ThreadRecorder.record(ThreadRecorder.java:156)
        at com.android.builder.profile.ThreadRecorder.record(ThreadRecorder.java:120)
        at com.android.build.gradle.BasePlugin.lambda$createTasks$1(BasePlugin.java:603)
        at org.gradle.internal.event.BroadcastDispatch$ActionInvocationHandler.dispatch(BroadcastDispatch.java:93)
        at org.gradle.internal.event.BroadcastDispatch$ActionInvocationHandler.dispatch(BroadcastDispatch.java:82)
    ...

* Where:
Script 'E:\cordova\helloworld\platforms\android\CordovaLib\cordova.gradle' line: 117

* What went wrong:
A problem occurred evaluating root project 'android'.
> Unable to determine Android SDK directory.

* Try:
Run with --stacktrace option to get the stack trace. Run with --debug option to get more log output.

BUILD FAILED

Total time: 5.186 secs