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.

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。

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You must be sure that, if you're doing a right rotation, the top node has a left child.

必须要确保，如果做右旋，顶端节点必须有一个左子节点。

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Science editor Alan Boyle's blog: A laser attack on a lymph node provides the drama behind the top-rated video in Nikon's 2012 Small World in Motion competition, which celebrates time-lapse movies made on a microscopic scale.

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