定义了两个变量和三个函数  要求func1循环n1次 func2循环n2次 之后调用func3.
如果能在一个异步函数中实现，额外加分
int n1 = arc4random() % 1000;
int n2 = arc4random() % 1000;

- (void)func1 {
    self.count++;
}
- (void)func2 {
    self.count--;
}
- (void)func3 {
    NSLog(@"a%d %d", __LINE__ ,self.count);
}

因为好久没碰iOS了，及其生疏 居然脑袋里一片空白 这明摆着dispatch_group啊
害，gcd组太好实现了，就不说了 看下怎么在一个异步函数中实现吧
其实用到了信号量 ，用信号量来拦截前两个异步任务 信号量恢复0后再去执行func3，重点其实在于第三个函数前的两个wait函数 一定要wait两次，才能拦截两个异步任务。
大胆猜测一下，系统的gcd组应该用的也是信号量吧 哈哈哈

dispatch_semaphore_t seam = dispatch_semaphore_create(2);
    dispatch_queue_t queue = dispatch_queue_create("lion", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(dispatch_get_main_queue(), ^{
        
        dispatch_semaphore_wait(seam, DISPATCH_TIME_FOREVER);
        dispatch_async(queue, ^{
            for (int i =0; i<n1; i++) {
                [self func1];
                NSLog(@"n1: %d", i);
                
            }
            dispatch_semaphore_signal(seam);
        });
        dispatch_semaphore_wait(seam, DISPATCH_TIME_FOREVER);
        dispatch_async(queue, ^{
            for (int i = 0; i < n2; i++) {
                [self func2];
                NSLog(@"n2: %d", i);
                
            }
            dispatch_semaphore_signal(seam);
        });
        
        dispatch_semaphore_wait(seam, DISPATCH_TIME_FOREVER);
        dispatch_semaphore_wait(seam, DISPATCH_TIME_FOREVER);
        [self func3];
        dispatch_semaphore_signal(seam);
        dispatch_semaphore_signal(seam);
    });