堆栈的顶存储在 result 中，使用 CAS 把 top 位置更新为 top->next 并返回适当的数据。
The top of the stack is stored in result, and you use CAS to update the top location with top-<next and return the appropriate data.
这样，即使线程b在线程A试图弹出数据的同时修改了堆栈顶，也可以确保不会跳过堆栈中的元素。
With this arrangement, even if Thread B has modified the top while Thread a tries to pop, you're sure that no elements in the stack are skipped.
push方法观察当前最顶的节点，构建一个新节点放在堆栈上，然后，如果最顶端的节点在初始观察之后没有变化，那么就安装新节点。
The push method observes the current top node, constructs a new node to be pushed on the stack, and then, if the topmost node has not changed since the initial observation, installs the new node.
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