leetcode No1114. Print in Order

Posted 2022-06-11 Dufre.WC

Question

Suppose we have a class:

public class Foo 
  public void first()  print("first"); 
  public void second()  print("second"); 
  public void third()  print("third"); 

The same instance of Foo will be passed to three different threads. Thread A will call first(), thread B will call second(), and thread C will call third(). Design a mechanism and modify the program to ensure that second() is executed after first(), and third() is executed after second().

Example 1:

Input: [1,2,3]
Output: "firstsecondthird"
Explanation: There are three threads being fired asynchronously. The input [1,2,3] means thread A calls first(), thread B calls second(), and thread C calls third(). "firstsecondthird" is the correct output.

Example 2:

Input: [1,3,2]
Output: "firstsecondthird"
Explanation: The input [1,3,2] means thread A calls first(), thread B calls third(), and thread C calls second(). "firstsecondthird" is the correct output.

Note:

We do not know how the threads will be scheduled in the operating system, even though the numbers in the input seems to imply the ordering. The input format you see is mainly to ensure our tests’ comprehensiveness.

Algorithm

leetcode越来越强大了，也有多线程的题目了，不得不说这些题目确实对理解多线程很有帮助。这一题是按顺序打印字串，看到按顺序这三个字。

C++多线程同步的几种方式

Code

semaphore
C++11 和 Boost.Thread 都没有提供信号量，简单来说，就是信号量太容易出错了（too error prone），通过组合互斥锁（mutex）和条件变量（condition variable）可以达到相同的效果，且更加安全。更详细的讲解请看：C++ 并发编程（六）：信号量（Semaphore）。

• 执行用时 :156 ms, 在所有 cpp 提交中击败了23.83%的用户
• 内存消耗 :9 MB, 在所有 cpp 提交中击败了100.00%的用户

#include <semaphore.h>
class Foo 
public:
    sem_t sem_one;
    sem_t sem_two;

    Foo() 
        sem_init(&sem_one, 0, 0);    
        sem_init(&sem_two, 0, 0);    
    

    void first(function<void()> printFirst) 
        // printFirst() outputs "first". Do not change or remove this line.
        printFirst();
        sem_post(&sem_one);
    

    void second(function<void()> printSecond) 
        sem_wait(&sem_one);
        // printSecond() outputs "second". Do not change or remove this line.
        printSecond();
        sem_post(&sem_two);
    

    void third(function<void()> printThird) 
        sem_wait(&sem_two);
        // printThird() outputs "third". Do not change or remove this line.
        printThird();
    
;

mutex

• 执行用时 :152 ms, 在所有 cpp 提交中击败了23.97%的用户
• 内存消耗 :9 MB, 在所有 cpp 提交中击败了100.00%的用户

class Foo 
public:
    pthread_mutex_t mutex1;
    pthread_mutex_t mutex2;

    Foo() 
        pthread_mutex_init(&mutex1, NULL);
        pthread_mutex_init(&mutex2, NULL);

        pthread_mutex_lock(&mutex1);
        pthread_mutex_lock(&mutex2);
    

    void first(function<void()> printFirst) 
        // printFirst() outputs "first". Do not change or remove this line.
        printFirst();
        pthread_mutex_unlock(&mutex1);
    

    void second(function<void()> printSecond) 
        pthread_mutex_lock(&mutex1);
        // printSecond() outputs "second". Do not change or remove this line.
        printSecond();
        pthread_mutex_unlock(&mutex2);
    

    void third(function<void()> printThird) 
        pthread_mutex_lock(&mutex2);
        // printThird() outputs "third". Do not change or remove this line.
        printThird();
    
;

condition variable

class Foo 
public:
    mutex m;

    condition_variable cv1;
    condition_variable cv2;

    bool second_ready = false;
    bool third_ready = false;

    Foo()  
    

    void first(function<void()> printFirst) 
        unique_lock<mutex> lk(m);
        // printFirst() outputs "first". Do not change or remove this line.
        printFirst();
        second_ready = true;
        cv1.notify_one();
    

    void second(function<void()> printSecond) 
        unique_lock<mutex> lk(m);
        cv1.wait(lk,[this]()return second_ready == true;);
        // printSecond() outputs "second". Do not change or remove this line.
        printSecond();
        third_ready = true;
        cv2.notify_one();
    

    void third(function<void()> printThird) 
        unique_lock<mutex> lk(m);
        cv2.wait(lk,[this]()return third_ready == true;);
        // printThird() outputs "third". Do not change or remove this line.
        printThird();
    
;

promise/future

class Foo 
public:
    promise<int> p1,p2;
    Foo() 
        
    

    void first(function<void()> printFirst) 
        
        // printFirst() outputs "first". Do not change or remove this line.
        printFirst();
        p1.set_value(0);
    

    void second(function<void()> printSecond) 
        p1.get_future().wait();
        // printSecond() outputs "second". Do not change or remove this line.
        printSecond();
        p2.set_value(0);
    

    void third(function<void()> printThird) 
        p2.get_future().wait();
        // printThird() outputs "third". Do not change or remove this line.
        printThird();
    
;