Rather than wait for the other thread to finish, the current thread can "help" it out by finishing the operation for it by moving the tail pointerforward (b).
不必等候其他线程完成，当前线程就可以“帮助”它完成操作，把尾指针向前移动(B)。
2
When clicking the backward and forward buttons for the application, we won't be adding new events to the stack but we will be moving the pointer within the stack.
单击应用程序的后退和前进按钮时，不会在堆栈中添加新的事件，而是移动堆栈的指针。
3
If we don't need to do a GC, allocation is 1 moving a pointerforward and 2 clearing the memory for the new object.