哲学家进餐（力扣）

涉及到操作系统中的多线程并发操作

知识点

 1.  Semaphore(信号量)，维护一个许可集，同一时间最多允许多少线程去访问资源

     线程可以通过acquire()拿到许可，或release()归还许可

如果许可全部发放分配，则其他线程进入等待状态

2. ReentrantLock[]，可重入锁，类似于synchronized

独占锁且可重入的

class DiningPhilosophers {
   private final ReentrantLock[] lock = {
          new ReentrantLock(true),
          new ReentrantLock(true),
          new ReentrantLock(true),
          new ReentrantLock(true),
          new ReentrantLock(true),
       };
    private Semaphore limit = new Semaphore(4);
    public DiningPhilosophers() {
    
       
    }

    // call the run() method of any runnable to execute its code
    public void wantsToEat(int philosopher,
                           Runnable pickLeftFork,
                           Runnable pickRightFork,
                           Runnable eat,
                           Runnable putLeftFork,
                           Runnable putRightFork) throws InterruptedException {
        int l = philosopher;
        int r = (philosopher+1)%5;
        limit.acquire();
        lock[l].lock();
        lock[r].lock();
        pickLeftFork.run();
        pickRightFork.run();
        eat.run();
        putLeftFork.run();
        putRightFork.run();
        lock[l].unlock();
        lock[r].unlock();
        limit.release();

    }
}