说明：  这是一个关于线程间的同步问题。之所以每次会有不同的答案，是因为当两个线程并发执行时，程序执行的顺序是交叉进行的，导致程序的执行出现了不可在现性。有时会出现一个结果，有时出现两个结果，是因为两个线程不一定同时不为零。
(This is a thread on the synchronization problem. Each has a different answer, because when two threads with the implementation, the program is the implementation of the order for the cross-led process of the implementation is a must in nature. Sometimes there will be a result, sometimes two, it is not necessarily because the two threads while not zero.)