学计算机的那个

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多线程面试题

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面试题 1

假设我有一个类

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@interface OrderPint : NSObject

- (id)init;
- (void)first;//打印"first"
- (void)second;//打印"second"
- (void)third;//打印"third"
- (void)run:(NSString *)order;

@end

三个不同的线程ABC将会共用一个 OrderPint 实例
线程A 会调用 first 方法
线程B 会调用 second 方法
线程C 会调用 third 方法

请完善OrderPint，让 first second third 按照指定的顺序被打印

例如
输入 @”123” 打印

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2
3
first
second
third

输入 @”321” 打印

1
2
3
third
second
first

Demo

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#import "OrderPint.h"


@interface OrderPint ()
@property (nonatomic) dispatch_semaphore_t semaphore;
@end

@implementation OrderPint

-(id)init{
    _semaphore  = dispatch_semaphore_create(1);
    return [super init];
}


//打印"first"
- (void)first{
    NSLog(@"%@:first",[NSThread currentThread]);
}
//打印"second"
- (void)second{
    NSLog(@"%@:second",[NSThread currentThread]);
}

//打印"third"
- (void)third{
    NSLog(@"%@:third",[NSThread currentThread]);
}

- (void)run:(NSString *)order{
    if (order.length == 3){
        
        dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
        dispatch_group_t group = dispatch_group_create();
        dispatch_group_async(group, queue, ^{
            [self transferToFunction:[order substringWithRange:NSMakeRange(0, 1)]];
            [self signal];
        });
        
        [self wait];
        dispatch_group_async(group, queue, ^{
            
            [self transferToFunction:[order substringWithRange:NSMakeRange(1, 1)]];
            [self signal];
          
        });
               
        [self wait];
        dispatch_group_async(group, queue, ^{
            [self transferToFunction:[order substringWithRange:NSMakeRange(2, 1)]];
            [self signal];
        });
    
    }
}

-(void)transferToFunction:(NSString*) str {
    if ([str isEqualToString: @"1"]){
        [self first];
    }else if ([str isEqualToString: @"2"]){
        [self second];
    }else if ([str isEqualToString: @"3"]){
        [self third];
    }
}

- (void)wait {
    dispatch_semaphore_wait(_semaphore, DISPATCH_TIME_FOREVER);
}

- (void)signal {
    dispatch_semaphore_signal(_semaphore);
}
@end