多线程笔试题

[ShannonChenCHN]

题库：https://leetcode-cn.com/problemset/concurrency/

[ShannonChenCHN]

线程数数限制和事件回调

现在有多个任务，最多只允许使用三个线程，每个任务完成后需要知道当前还剩多少个没有执行完。

解法一：NSOperationQueue

解法二：并行队列 + 信号量 semaphore（参考 QSDispatchQueue）

解法三：创建指定数量的 Serial Queue（参考 YYDispatchQueuePool）

[ShannonChenCHN]

1114. 按序打印

三个不同的线程 A、B、C 将会共用一个 Foo 实例。

• 一个将会调用 first() 方法
• 一个将会调用 second() 方法
• 还有一个将会调用 third() 方法
  请设计修改程序，以确保 second() 方法在 first() 方法之后被执行，third() 方法在 second() 方法之后被执行。

int main(int argc, const char * argv[]) {
    
    Foo *obj = [Foo new];
    
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_async(queue, ^{
        [obj first];
    });
    
    dispatch_async(queue, ^{
        [obj second];
    });
    dispatch_async(queue, ^{
        [obj third];
    });
    
    sleep(5);
    
    printf("\nDone!\n");
    
    return 0;
}

解法一：信号量 semaphore

@interface Foo : NSObject

- (void)first;
- (void)second;
- (void)third;

@property (nonatomic, strong) dispatch_semaphore_t semaphore1st;
@property (nonatomic, strong) dispatch_semaphore_t semaphore2nd;

@end

@implementation Foo

- (instancetype)init {
    self = [super init];
    if (self) {
        self.semaphore1st = dispatch_semaphore_create(0);
        self.semaphore2nd = dispatch_semaphore_create(0);
    }
    return self;
}

- (void)first {
    
    NSLog(@"first");
    dispatch_semaphore_signal(self.semaphore1st);
}

- (void)second {
    dispatch_semaphore_wait(self.semaphore1st, DISPATCH_TIME_FOREVER);
    NSLog(@"second");
    dispatch_semaphore_signal(self.semaphore2nd);
    
}

- (void)third {
    dispatch_semaphore_wait(self.semaphore2nd, DISPATCH_TIME_FOREVER);
    NSLog(@"third");
}

@end

解法二：NSCondition

@interface Foo : NSObject

- (void)first;
- (void)second;
- (void)third;

@property (nonatomic, strong) NSCondition *condition1st;
@property (nonatomic, strong) NSCondition *condition2nd;

@end

@implementation Foo

- (instancetype)init {
    self = [super init];
    if (self) {
        self.condition1st = [[NSCondition alloc] init];
        self.condition2nd = [[NSCondition alloc] init];
    }
    return self;
}

- (void)first {
    
    NSLog(@"first");
    [self.condition1st signal];
}

- (void)second {
    [self.condition1st wait];
    NSLog(@"second");
    [self.condition2nd signal];
    
}

- (void)third {
    [self.condition2nd wait];
    NSLog(@"third");
}

@end

[ShannonChenCHN]

1116. 打印零与奇偶数

解法一：信号量 semaphore

解法二：NSCondition