android 点击通知栏返回应用 ，非启动一个新Activity

Intent intent = new Intent(this, MainActivity.class);
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_SINGLE_TOP);

public Notification createNotification() {
Notification nf = new Notification(android.R.drawable.stat_sys_download, "正在为您下载...", System.currentTimeMillis());

nf.flags = Notification.FLAG_ONGOING_EVENT;
nf.contentView = new RemoteViews(getPackageName(), R.layout.notification_download);
setNotificationInfo(nf, 0, "..M", null, true);

Intent intent = new Intent(this, MainActivity.class);
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_SINGLE_TOP);

nf.contentIntent = PendingIntent.getActivity(this, 0, intent, PendingIntent.FLAG_UPDATE_CURRENT);

return nf;
}