How to show dialog yes/No before leaving the app via Back button ?

private boolean isLastActivity() {
   final ActivityManager am = (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
    final List<RunningTaskInfo> tasksInfo = am.getRunningTasks(1024);

    final String ourAppPackageName = getPackageName();
    RunningTaskInfo taskInfo;
    final int size = tasksInfo.size();
    for (int i = 0; i < size; i++) {
        taskInfo = tasksInfo.get(i);
        if (ourAppPackageName.equals(taskInfo.baseActivity.getPackageName())) {
            return taskInfo.numActivities == 1;
        }
    }

    return false;
}

You wil also require below permission in manifest....

<uses-permission android:name="android.permission.GET_TASKS" />

Thus in your Activty you can just use the following:

public void onBackPressed() {
    if (isLastActivity()) {
         showDialog(DIALOG_EXIT_CONFIRMATION_ID);
    } else {
         super.onBackPressed(); // this will actually finish the Activity
    }
}